Wifi deteriorating over time

Can anyone give me help on how to stop my wifi from deteriorating, it barely lasts a day, in the connection strength, 79% is the point that I cant really use internet anymore without massive lag (its at 85-90 when I powercycle). My connection point is in my workshop about 20-30 yards away, I am using a Netgear router(Belkin does the same), and the connection strength goes down constantly, I have tried power cycling, and resetting every setting I can think of.

You're experiencing some sort of interference. Here's an influence chart:

Type of Barrier     Interference Potential
Wood: Low
Synthetic material: Low
Glass: Low
Water: Medium
Bricks: Medium
Marble: Medium
Plaster: High
Concrete: High
Bulletproof glass: High
Metal: Very high

Do your research, eliminate some interference or reposition your access point.

would other connections in the area interfere?