Pontiac Repair: 1995 Grand Am Oil Oil pressure switch & head gaskets, oil pressure switch, coolant level

Question
Hello,
1995 Grand Am 3100 automatic. My Daughter bought it used 1 week ago. 160,000 miles. Engine swapped at some point (not by our seller). Unknown milage on engine. Seller had installed new radiator 1 month ago. Car ran great. strong,no smoke no ticks, drove very nice,tranny felt firm. I gave it a thumbs up. We changed oil & filter. 5 days later (running great)shuts off car. Restarts 15 min later runs on maybe 4 cylinders/ very rough almost doesn't start/ barely keep running. Rad. hoses rock hard. motor clatters. Oil pressure light is on while running. Oil level is normal(not over full like coolant in oil) I put a pressure gauge on the air bleeder fitting. Just cranking/running for a few seconds cold puts 10-20lbs pressure on the gauge. Probably head gasket? there are no external leaks & no extra fluid level on the dip stick  but why oil light at same time? connected or seperate issues? I can't find the pressure switch. (no gauges just dummy lights). I looked by the oil filter adapter pad but can't find it. The AC comp. is right there I want to hook up a live gauge but where can I tie in.     
Thanks,
Scott

Answer
Hi
How is the coolant level? Is it OK?
color? Bright orange (if dexcool) or green (if glycol).
The best way to check for head gasket would be either pressureing up the Cooling system (can get coolant system testers loan at AutoZone) or by using Block tester fluid (A Blue fluid that turns green/yellow with presence of Exahust gasses in coolant)
The oil pressure is probabally just flutuating due to the misfiring of the engine.

I, personally, would FIRST consider checking the ignition side of the engine as opposed to the mechanical side also.
Take (one at a time) the plug wires off the coils (on the top of engine rearward)
If ANY are rusted, there MAY BE your problem.
You'll need to replace the faulty coils (with rusted towers) and the plugs.
It may not be a bad idea to preform a complete major tune up....Plugs, wires, fuel filter, air filter.

Also FYI the Oil pressure switch is located just above the A/C Compressor.
I hope this all helps.
Please follow post back, if needed.
Pawl