My truck lags and loses power during acceleration. It also tends to die when put into drive or reverse.?

I have a 1993 Ford Bronco and I am thinking it is a fuel problem, possibly the pump? All i know is that I do not want to have to pay for the pump to be replaced and find out its something else. Also when I'm cruising freeway speeds i noticed it doesn't really have an in between engine speed, it will either run at kind of the minimum to keep the truck moving at the same speed, or it will kick it down a gear and rev up considerably. The last part to this is that the "check engine" light comes on randomly and goes off randomly. I don't know if this is connected, but someone mentioned possibly the O2 sensor setting that off, and possibly creating a too rich or too lean fuel mixture? No idea, would appreciate some input! Thank you very much.