I found that my WRT54G running either Tomato or DD-WRT was very channel dependant - I would get all manner of flakiness on some channels and decent reliability on others. Is your WRT54G set to 'Auto' channel? It might have switched to one which is flaky. I'd recommend setting a specific channel manually.

Also, I found issues with Broadcom based wireless APs such as the WRT54G and Intel 2200BG clients (the old original 'Centrino' wireless card). But that doesn't really explain why things used to work okay and now don't, unless you now have one on your network or something.