engine cuts out while cruzing

I'm having a problem that has me stumped, hopefully someone with a lot more knowledge about the car reads this and can help.
I've got a 98 Cavalier, 2.2 L with a manual transmission and 157k miles that i can feel it cutting out or missing only when i'm maintaining a speed. I feel it a lot more going downhill, but it is definately there when i'm maintaining 55, 65 or 75 mph. It doesn't do it when there is a load on the engine, like when i'm accelerating or going uphill, only when i'm maintaining a constant speed.
I've checked my plug wires, all look good. my plugs are platnum and have only about 40k miles on them. At my last oil change I did an engine flush, and changed my air and fuel filters to no avail.
Has anyone ever had this happen to them? What did you do to fix it?
Thanks

Could be the fuel pump. When I had my 2.2, I was crusiing, and it cut out, kept doing it, and then just died and wouldn't start, it was the fuel pump.