Its called dynamic rate shifting, the further from the router/ap you get it drops the speed to maintain a reliable connection. Other reasons are bit errors caused by interference or a variety of other causes. If the error rate is high it will continue to drop all the way to 1mbs until it has a reliable connection. Might try repositioning your ap or the antennas.
Most systems have a time setting/delay for hard disk/system/screen hibernation/shutdown etc. go into power settings and set everything to never turn off, try it for that night and see what happens.
There is a definite problem with the system hibernating while connected, and then powering back up with the old settings.. Sounds like you may be running into that when it sits overnight/long enuf to go into power save/hibernation etc mode. Laptops are usually set at a very short active time before power save, but even desktops power save after a few hours.
I've not been around long enough to know what brand that model number applies to but...
In googling around for the better part of a month before buying a WiFi setup about 6 weeks ago I found lots and lots of problem with wifi gear overheating. Seems the thermal design is waaaay out there. My LinkSys USB client unit is that way. It'll run fine on the desk but let even a ray of sunlight hit the case and it's outta here. Same if I leave it sitting on top of my laptop. The big ASIC chip is the culprit in my unit. I foresee a hole in the case with a heatsink poking thru :-)
I'm having a similar problem with my Netgear WG511T CardBus card in my laptop. This Dell Inspiron 4150 runs rather hot when it's busy, warm enough that the card will shut down after awhile. If I leave the top CardBus slot empty and aim a fan at it, things are fine.
Check your location and see if things are too hot. Anything about room temperature should be suspect.
If it's not temperature and the latest drivers don't fix the problem, take it back. If that doesn't fix the problem, take it back again and get another brand.
Thanks for the info. Theres another problem is that after a night long, although WiFi is well connected between PC and AP with very good signal, I couldn't get connected to the internet untill I turn off and back on the router/ap (DI-724P+) again. Is there any time setting i miss out which stop the router from working all night long ?