This is strange, and definitely a PC problem, but y'all are smart, so I thought I'd give it a go in here.
This happened with my TP2 and now with my DX (had it for two days - AWESOME phone!!). I plug up the phone via USB, and it begins to charge. Usually I'm also tethering. After an hour or so, the PC no longer recognizes the phone, and the PC loses the internet connection, though the phone claims it's still connected, though the charge light is off. Restarting the PC fixes the problem for another hour or so. Other PCs do NOT do this.
WHAT the heck? Is my PC power supply dying? This is very strange.
If it helps, I've been tethering with my TP2 and this problem only recently begun, but I always noticed that though the phone said charging and the LED was on and the battery showed the charge creeping up to full, the phone was not charging properly - the charge would be depleted very quickly when unplugged. I think there was just enough charge to fool the phone into thinking it was charging, even up to thinking it was full, so when I hooked it up to the wall wart overnight, it didn't try to charge.
My brain hurts - sure appreciate any ideas!