Oil Light Coming On

I have a '97 Cavalier and the oil light comes on after I drive on the highway. Once I accelerate it goes out ,but comes back on when I slow down. I had to change the head gaket and changed the Oil Pressure Switch out at the same time. The problem still hasn't gone away. Is there another sensor that I should be looking for? I asks the counter person at AutoZone, but they were no help. They said that they needed a part name. I checked the oil level and it was fine. The last time it came on I was about a 1/2 quart low and filled it up. I can drive all day in the city without the light coming on. Any help would be great.

When u took the head off did u take it to a machine shop and pressure test it? It could have a hairline crack. I would do a pressure test on each cylinder.