I also had the same problem with my 88 Grand Prix (stalling). We had taken it into a repair shop and they told us that the fuel injectors were plugged. I had trouble believing this, so I changed the fuel pump and fuel filter, after doing this I found the fuel pressure and flow to be adequate. The only thing really left was the fuel injectors; it seems that the small size of these fuel injectors (15 lbs. per hour.) makes them more likely to plug up. This is also seems to be quite common with other 2.8 liter Chevrolet V6's at around 130,000 miles. The cost for new fuel injectors is generally between $250.00 and $300.00 not including the labor, also this type of fuel injector is impossible to be cleaned completely. So your best bet is to have new fuel injectors installed.
The stereo is probably a busted or loose rubber band in the tape player, though I wouldn't bet my life.
Change your oil when it starts to stall, and it should perk up. I know that the ONLY time our Prix does that is when it needs new oil. EVERY SINGLE MECHANIC said it was crazy, but it has never failed to be the problem. This and our Prix has been beaten within an inch of its life on dirt roads and back alleys in CA for the last eight years, so it SHOULD be having problems by now.
I own a 1988 Grand Prix LE. I had stalling problems also I was however able to have the Pontiac dealer trace the problem to a faulty ground wire from the temperature sensor in the radiator. This new blue ground wire runs to the computer and has solved the stalling problem. However I now have a tachometer that gives me false high readings, but the car doesn't stall any longer.
Hope this may help some of you.