lack of performance and gas milage

[96altimagxe]

I have a 96 Altima and have recently been experiencing a problem. I've been getting terrible gas mileage and lacking power after the car warms up. The weird thing is if u play w/ anything ignition related, the problem clears up for a little while. It seems like the timing gets really screwed up the further I drive. I know the OBD2 ecms automatically set the timing. I was getting a code for the knock sensor, so i relocated the new one which stopped the computer from screwin things up more. That made the problem a little better. I changed plugs and wires 3 times, changed cap and rotor, pulled distributor off and clean the shaft, checked the primary coil resistances and they were all fine. If I pull the front O2 sensor off it actually runs a little better. I'm stumped on what the problem could be, Every time I advance the timing it runs a lot better for a while, then the ecm adjusts it back to a terrible setting. I would appreciate any help w/ this problem.

[96altimagxe]

I also dopped the exhaust to check for catback and that didnt help either.