one or two things .... when computers slow down or dont work continuously thre can be many reasons:
1. It could be tha the ISP is doing maintenace or having problems
2. It could be that your router needs to be rebooted as well as the computer.
3. It could be overheating in the computer or router (are they well venilated and clean inside with no dust buildup on vents or heat sinks
4It could be virus but if you have good AV no probs
5. It could be failing componenets like memory etc but that often needs specialised equipment.
6. This sounds stupid but I had similiar problem with another wirelzss network giving a very high signal and mine trying to use that as well! Solved by telling it not to accept any other than the default network.
7. Microwaves (from cookers) can reduce signal strenth as can any metal or wire physical obstructions in hte way (bit like mobile pohones dropping out when out of range or bhind a hill etc;
Best of luck
M