1998 Pontiac Grand Am oil pressure light on occasionally

Electrical problem
1998 Pontiac Grand Am Front Wheel Drive Manual 94000 miles

My car has been running well since the glutch got repaired in April, 2007. A few weeks ago, the oil pressure light suddently came on and the engine stopped immediately. Afterwards, the chance for the oil pressure light to come on is growing. Last week on Nov. 2, 2008, the engine oil was replaced in Walmart to see if the problem goes away or not. Unfortunately, the situation is getting worse. The oil pressure light will come on more frequently. Furthemore, it is hard to start the car smoothly after the oil pressure light come on and I restart the engine.

More specifically, when the problem happens (the oil pressure light comes on; the engine seems to stop working; the steering wheel becomes stiff), I immediately turn on the alert light, turn the key into "off" status and restart the engine, and all settings are correct then. After that, when I pull my left foot off the glutch and meanwhile push the accerator pedal, the speedometer does not turn as usual till I push the accerator hard. Also, it seems that the ABS light will suddently come on very often after the "oil pressure light" problem happens.

Note: I have been using the oil change service. And the oil type is SuperTech 10/30.

My question: how to precisely classify the problem and accurately pinpoint the cause for this problem? Can this problem be easily fixed? Which kind of shop should I go to? Transmission shop?

thanks very much!