Chevy 350 engine problem to all you smart folk.

hi.
Our malibu which has a 350 is being quite bizarre. One day as we were out riding, the engine revved WAY down, then back up, then back down again, and back up until it just killed. after this it would just continue when you got up to speed. I changed out the fuel filter, which was pretty dirty. after this, it seemed to work pretty good until you were up to full throtle. So we just packed it up on the trailer and headed back, I figured there was a little crap left over in the lines and all would be good.. well today we went back out and the same problem from the beginning is occuring.. as soon and you go faster than idle, it goes back to revving way down then back to your throtle speed... im wondering if this is possibly a fuel pump?? im not sure.. could it be electrical.. I know a lot of you guys are brilliant with this motor, please help a fellow out thanks a lot!