Low Pressure Light Comming On???????

I have some what of a problem. I have a 1999 intrepid with original motor(75,000 k/m), and for some reason, when i drive it for oh i dont know, 10 - 15 minutes, the low pressure light comes on. I shelled out 70 bucks for the dealer to check it out and they told me nothing was wrong. When i got the car back, not even 20 minutes into driving the light came on again. This was after a freash oil change as well. What a waste of 70 bucks. I called them again today and explained it to the guy, fix the problem already, i even asked him if they started the car when they ran it on the diagnostick machine,lol. So he went and talked to the tech. He informed me that it was either the sensor in the block(wouldnt surprise me), or the oil pump. I'm taking it in on Mnday for yet, probably anotehr waste of 70 bucks, or more. 1 more thing, i was wondering if anyone else has had to change their oil cap. I have had to change it twice since i have owned it from new. I find its hard on belts as well. If any one can shed some light on this for me, please do, i would greatly appreciated, seeing as though the dealer havent a clue as of yet. Thanks