There is difference between signal strength and signal quality. You can have a very strong signal but be getting huge number of errors because of interference. There are some wireless cards that display both strength and quality or error rates.

No easy way to fix this. Standard change the channel and see if its better. Many things can cause interference. You would have to think if you had any new devices in your house lately that use wireless. You could also turn off all but one of your devices and test. You may have some new software sending junk out. Of course it could just be a new router at a neighbors house on the same channel.