The way I caused mine to do this was definitely non-hardware.

I'm slightly vague on exactly whar happened, but I flashed one of the images and then restored one of the files that was saved using Backupmenu!

Then my N900 ALMOST worked fine, except I started getting the telephony message up to every five seconds. I could do everything but use the phone. I tried many things to fix it.

Only reflashing the eemc and fiasco images in Linux solved the problem.

iam having the same problem, when i move the phone quickly or put it roughly on table it looses the network, i tried taking out the antenna and pulling the pins up but no luck, checked the sim slot, i noticed the loss happens when i hit the top of the phone with my finger, finally, sent it to nokia care and waiting.

i followed these instructions,

Experienced users still may want to follow the rootfs-then-eMMC scheme, which doesn't need the battery removal, but is way less "fool-proof" wrt accidental boots. A boot in this context is defined as any activity of the device (flashing LED, some display on screen) that doesn't result in and stop at the NOKIA boot screen with USB icon. Especially "charging" actually also implies a boot of the system, and there's no lower limit for duration of such an accidental boot, 5s are more than enough to ruin your freshly flashed system.

http://wiki.maemo.org/Updating_the_f...MC_in_the_N900

^ btw speaking of flashing, you'll know if it's a hardware issue if during the flashing it spits out:

Sending and flashing rootfs image (185856 kB)...
100% (185856 of 185856 kB, avg. 11460 kB/s)
Finishing flashing... done
CMT flashing failed

that fail is the disconnect/lost of contact between the antennas, or thats what im assuming

Geeze, that section of the doc seems to have been rewritten several times.

I just installed the flasher and downloaded the correct eemc and fiasco imades. Then WITHOUT EVEN PLUGGIN IN OR TURNING ON MY N900 I ran

flasher-3.5 -F RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin -f

Then I got the message about suitable USB not found.

Then and only then I plugged in the USB cable to my computer and the N900 and flashing started immediately.

After that completed successfully I did the same with the fiasco image, and all was fine.

Nokia called, my phone is a total loss, they can't figure it out either. I was offered an N8 and I refused, in my view N8 is below my expectations for a phone, I saw several and they didn't work right. We'll be re-negotiating again tomorrow, I'll have to pick a phone from their current lineup.

I'll probably get an E7, it has a hw keyboard and similar hardware to the N900, but as we all know, there will be no more M5 phones out, and it's likely Maemo will not be seen again in a release. Their announced MeeGo phone is so far off course it's not funny. No hardware keyboard, for starters.

I asked them to look and ask around for about a week before calling me with "no replacement". I guess that's it, then. N900 is officially dead.

Well, folks, it's been fun. Ride's over.

I was finally getting attached to the lil' bugger, I've just grown accustomed to having a system image on-device, overclocking and so on. Guess it's back to phones from now on.

On the bright side, better camera, actual maps, an actual phone. So, you know. Not all bad.

Take care of your N900s. They don't make them like that any more.

P.S. Guess I shouldn't have made fun of E7 up there.

Just thought I would post an update.
Full re-flash didn't solve the problem for me, so I put my Vodafone (n900) sim card into my android phone, so as not to miss calls. Whilst browsing, I got a similar message about connectivity issues, and also, the Orange sim card I put in my n900, has been fine. So it looks like my problem was with the sim card. I now have ordered a new Vodafone sim card, and hopefully I won't be seeing this issue anymore.
EDIT: Wrong. New simcard doesn't stop this issue. Looks like I'm going to have to send it back. Really annoying.

i dismantled and checked the antenna connections and lifted the tabs a bit and nothing changed. my sim works fine in other phones.

any other suggestions?

Exactly the same issue as the rest of you - been ocurring on and off for past 3 weeks but getting more and more frequent. Reboot clears it for a while but its not more than an hour or two now before its back again.

Sometimes, like @ndi said, everything looks ok but texts don't get through and it only becomes obvious there's a problem when i want to make a call or send txts

Decided only option is to claim under warranty but I don't have original proof of purchase.

Has anyone else managed to get service from Nokia without proof of purchase? (their website confirms the handset is still under warranty from the IMEI no.)

If you can find a Nokia Service Point they will send it to Nokia ... they did so in my case. They didn't want o see any inquiry or something.
But beware ... they could not repair it and they have no N900 so i got an E7. I'm now trying to contact amazon (i bought it there) but they also have no N900 ... :-(

Maybe I'll get an N9 .... ha ha ha ;-)