1991 cavalier wagon issues

This is my my first post so forgive me if it goes badly.

I have a 1991 cavalier wagon with a whole 95 horse that I purchased for $500 and was told it needed nothing. At the time she had 136,000 on the clock but had just seen a new headgasket and a TC cluch silenoid. 3 months into owning it I started tracking the mileage and found I am only getting 20 miles to the gallon. To top it off I have had to replace a dozen parts just to keep it on the road and the problems still stayed with the car. I poured some carb cleaner in the tank today and can't find or hear any vacuum leaks. Here's a quick list of my issues.

1. Crappy Gas mileage

2. During some startups it won't stay started w/o my foot on the gas.

3. Idle is a bit rough sometimes

4. Oil pressure gauge dances all over the place when I'm idling and yet is fully functional while I'm driving.

5. I get squealing/grinding noise coming from the right front tire when the wheels cut all the way to the right.

In the last 30 days the car has had the following parts installed.

New Calipers, rotors and pads on the front
Battery
Starter
Radiator
4 mounts
serpentine belt,
plugs+wires
fuel filter
New tires and a front end alingnment
catalytic convertor