My 1 year old n800 has started showing this problem yesterday:

after the screen has been off varius hours, when it turns on I get horizontal black lines running up and down the top third and the bottom third of the screen, but only if the background image is mostly dark (e.g. the unlock keypad or the home screen with an image), no lines with an app open (even looking at hackaday, which is black!) or with the home screen set to a white background.

Eventually the lines disappear completely, which makes me think it's a faulty electrolytic capacitor somewhere, but it could be anything else.

I searched and I only found people reporting vertical lines, and that issue was fixed by reflashing, but I think this is a different one.

I now discovered that the black lines are only where the right edge is dark, so no problem with hackaday due to the white scrollbar.

hmm i sometimes get this with a gray background and some bright colored graphic in the middle. i see faint horizontal dark lines but it doesn't bother me much. maybe it comes with age

Well, it doesn't bother me too much if it doesn't get worse and the screen remains useable.
How long have you been seeing those faint lines?

It is getting worse
Now it doesn't recover after a while.
edit: it eventually recovers, but it isn't the time the screen is on that matters, but use of wifi (or maybe the cpu load caused by it), so if it is a condenser it's probably one near the wifi chip.

So you are basically saying it is the temperature... WiFi use and loading CPU increase the internal temperature.

Could it be a bad connector/connection between the display and "motherboard"? Have you tried to tap/shake the device to see if that makes a difference?

Yes, that's my guess

Exactly
It could be, however I think the lcd connector is on the opposite side of the wifi chip. Mmmh.
Yes, I tried and it doesn't seem to make a difference.
I guess I'll have to open it to check

Well, it's definitely heat related, lack of heat actually: the problem is less pronounced if I keep the tablet in my pocket (keeping it warm) and it is more pronounced if I leave the tablet alone (now that ambient temperature has lowered).
Using an hair dryer makes the problem go away for a while, but that's hardly a solution.

Well, I finally found the required torx t6 screwdriver and I opened the tablet.
Unfortunately I think it's the screen itself that's broken: reseating the connector made no change, heating (with the hair dryer) the mainboard had no effect, the only change I noticed when heating/cooling the screen alone.
Now I don't know what to do: change the screen (which is at least 100€) or use it as it is while it lasts.

An update (a long message but I hope it is useful to people needing replacement parts).

Since my last message I did something stupid: I removed the bezel from the display itself to try and push the flat cable, suspecting a bad contact on the lcd side.

Well, the flat cable is extremely delicate (either that or I'm extremely clumsy), and while manipulating it I really broke it (the screen still worked, with the black lines, but the touch sensor didn't).

I contacted Nokia Spain through their online form to know where I could get my tablet repaired......and I'm still waiting for a reply.

Meanwhile I started looking online for a replacement screen. Tessco was out since they don't sell to end users.

I found one at a local online shop (impextrom) for 77€ plus shipping (8€) plus VAT, for a grand total of 98€. Their page showed that they had the display in stock.

I also found it at mytrendyphone, for 96€ plus VAT, but they have a price match option, so I told them I found it cheaper.

In a couple of days they lowered the price to 75€ plus VAT, but since the VAT is higher there (25%) than here (16%), I ordered from impextrom anyway.
Next day they told me that they have no stock and cancelled my order . I asked if they were going to have more stock...and I'm still waiting for a reply.

Meanwhile, even if I don't understand german, I checked the german page of mytrendyphone (meintrendyhandy), since the vat is somewhat lower in Germany. and I placed my order there.

For a strange glitch they didn't charge me neither the VAT nor the shipping (maybe because I chose the option to show the prices without VAT), though they charged a 2€ fee for paying with credit card. I told them I found it strange that they didn't charge VAT but they told me it's ok (it's sure it's ok for me).
I ended up paying 77€ total for the display.

It turns out they're based in Denmark (they sent the screen from there).
The order tracking page didn't work form me, neither in the german nor in the uk site, I had to use the danish or norwegian site to get a tracking number.

They sent it the same day by mail and it took a week to arrive.

It arrived today, they sent it in a simple padded envelope with very little bubble wrap inside, no box, not even a piece of cardboard to protect it from bending, nevertheless it was in good shape, I fitted it and it works.

Now I have a new, shiny, display, I hope it lasts more than the old one.

They also have the display for the n810 and other spare parts, just search n800 or n810, so check them if you need some spares....and click on the option to show prices without VAT