Wireless weirdness

Ok so for the past ~2 years I’ve used ATT DSL and a 2wire all in one router/wireless/modem, etc and the past week or so my wireless has been dropping constantly and slow(only 300-500k download). It worked almost flawlessly for a 2 years. The PC connected via ethernet is fine so I assume it’s not a DSL issue. If I move the router right next to a PC and connect via wireless that seems to work fine as well(no speed loss, etc). The way I need to get it setup is a PC on the 2nd floor connects direct then a PC in the basement uses the wireless. It’s about 30ft and through 2-3 walls. There are a lot of wireless networks around here. The only thing I can think of is a new one is messing mine up. Would putting a bridge most likely fix the issue, or is it something else?

Nothing has changed on my end AFAIK when the problem started. Sometimes the wireless will still work decently but you can tell it’s not as snappy and always slower. Other times it can’t even keep a connection for even 30 secs over multiple attempts. Many other times it will connect and be ok for roughly 5-15mins(albeit slower) then just drop for what seems to be no reason. Very rarely it will still connect and hang on for a hour or two. In the past I had no problems maintaining a wireless connection for hours upon hours.