1991 Oldsmobile 98 idle / gas mileage

Dug into it and found the wiring was bad to the crank and cam sensors. Fixed that and it ran but still had problems.
Found the magnet on the cam had fallen off so the sensor couldn't read. Had that fixed and at that time changed:
timing sprocket set, new magnet, water pump, spark plugs, fuel filter, front motor mount, coil pack and module, O2 sensor.

Car ran good for a bit and the "check eng" came back intermittently. Scanned again and O2 came up again. Changed under warranty. Stored the car over winter and when I got it out it seemed to work fine. Drove it awhile and it got up to 25 mpg on the highway. (AUG) Then after that it started to go down hill fast. The last fill up I got 13 mpg. I understand my commute is very short and that would effect mileage but not this badly. Also the idle drops and comes back up sometimes so violently that the car will die. This happens when not under load in park or neutral.

Seems like when you first start the car it runs OK but as it warms up it starts to have trouble. Reminds me of an older car with a carb that the choke is sticking. I know the O2 is not really in the mix at first, the computer tells the car what to do but after it is warm it runs real rich and it would seem to me like it would try to lean out the fuel ratio. I'm no mechanic so I'm relaying on what other mechanics have told me and a Snap-On scanner.