Well, for me the signal strength is excellent, and at the moment while I'm trying to figure this out the antenna is about 1 foot away from the router, however it would probably be otherwise unavoidable having the PC case block the antenna since it's we are using PCI adapters.
The speed is fine, it's just that for instance when I ping my dsl modem for a few minutes, every 30 seconds or so(sometimes more), a 1000ms+ spike crops up. Because of this, online gaming is damn near ruined altogether.
Antennas to close together can cause problems too. I set mine at the lowest power possiable to maintain an excellent signal strength.
It all most sounds like you are hitting a buffer full state. What is the lease time set for? Could it be needing refreshing?
Is there a cordless phone or other radio equipment around?
Have you run NetStumbler for any length of time to see if something is in the area. I have a stray that keeps tring to connect to my AP once every couple of weeks. Havn't been able to locate where its coming from.