95 Blazer 4.3 4x4 engine randomly bogging down/ killing gas mileage for the last 6 tanks. Live in TN so still fairly warm out. No rhyme or reason to when its happening. Nothing overwhelming, like hitting a strong head wind, or having a/c compressor kick on (not it, switch in dash broken, a/c hasn't ran in 2 years).

Plugs,wires,cap,button,egr,pcv,fuel filter,air filter all replaced when diagnosing a previous problem. Any ideas? I was thinking oxygen sensor, but leaning away from it after reading other post.

as side note on dash, anybody know the name of the switch i need. Air only comes out defrost and foot vents, regardless of setting, and a/c compressor will no longer turn on.