I think if you are getting the USB symbol the USB port is good enough, though you could get a cheap hot air reflow unit and try reflowing the USB solder points, you might try putting some lead % solder paste on the pads area to improve the quality of the solder joints.
I had something similar with a Nexus tablet, after resoldering I was able to do full speed USB vs erroring out.
But the problem might also be the EMMC going bad and the bootloader stage process goes bad before it can complete a USB connection or boot. Which means too bad, I have had a few of these which never become available for flashing, especially a few tries fomr the Chinese "New"(refurbished) N900s a few years back.

Why would the USB symbol be appearing and then disappearing straight away then?

The more I think about it the more I am convinced that if I could just get hold of a full BL-5J battery I might be able to get my phone going again.
But all 3 I have are dead and I have nothing to charge them up with. Everything I can find here in Brisbane that might help me charge them is either really expensive or would mean waiting weeks/months for something to arrive on a slow boat from China (or both) so I can't charge them.

If you need an external charger find a cheap e-cigarette or phone battery charger and solder on two jumper wires for the + and - terminals and figure out how to make them stick without bending the contacts, this will directly charge the battery externally. You can find plenty of youtube bits on doing this to charge batteries and other stuff. I used to have an OpenMoko phone which has a problem similar to the n900 dead battery - cant start bootloader to begin charging, I just hacked off an old USB mouse and gave the battery 5 min at 5v, it worked and the phone was able to begin charging when I put the battery back in.

I will be visiting some shops tomorrow including one that sells all sorts of electronic gear and has a few "universal" Li-Ion chargers listed on their website to see if they have any suitable chargers.
And I have posted on a local Aussie electronics forum asking if they have any leads on a suitable charger.

If I can find a charger that will do the job its better than just trying to feed current in directly and would give me an external charger I can use going forward.

http://talk.maemo.org/showpost.php?p...8&postcount=30

After buying an external charger yesterday and leaving one of my batteries charging all night I was able to get flasher to boot. Then I flashed Fiasco and installed backup-menu. Then I restored the BM backup I did before I started all this mess. After that I installed cssu-testing then I installed a few very specific packages on top of that (either .debs downloaded from cssu-devel or .debs build locally from cssu-git)

Now I have 3 BM backups that I am currently downloading to my PC for safety (one is from before installing cssu, one is just after installing cssu and the last one is after I installed everything)

And now I have an external battery charger that can charge BL-5Js (and other types of rechargeables including all kinds of Li-Ion packs)

Good to hear/read!

It would have been another loss due to such a small issue...

Glad you got it going again

Ok. I've decided to keep my N900s and repair the USB ports, as they are working when I've charged the batteries externally.
I have a good soldering station, with a thin solderer, a magnifying lens, and holders/clips to hold everything in place.
Before I start, I just want to know of any tips/tricks on how to do this. Both USB ports have come/dropped off, and I've purchased a stick of brand new USB Type2 micro-usb ports. All I need is a steady hand to solder these little suckers back on the m/board and if done properly, it should all work and I'll have 2 working N900s. One for sale (Australia) if I get both working too!
Any tips/advice on how best to do this?
Thanks