1996 Chevy Lumina random missfire codes/poor mileage

I suspect a fuel issue, but after testing pressure, the regulator, filter and pump, I am at a loss. The car send random missfire codes on several different cylinders. The pump, regulator and filter (just replaced) all check out as normal. The engine cuts out at higher RPMs with no load, and then fires back up, then cuts out over and over again. It will also start loping at various times and then run smooth again. The code pops up, then the engine runs fine and the code disappears. I am at a loss. Checked vacuum lines, repaired one, and now I am not sure what to check. Mileage has dropped from 27 to 16 over the last 6 months. No smoke from the tailpipe. Ideas?