It accelerates fine up to 55 mph but after that it crawls to 65 unless I romp on the gas and rev the engine to 2500-3000 rpm. Some of the things I have done recently to try to remedy this are; changed plugs and wires, changed the serpentine belt(old one was shredding), changed the thermostat(this was stuck open and the engine never reached operating temp), changed the oil/filter, and I seafoamed the engine, oil, and gas as suggested in other posts(smoked like a fiend). Power and acceleration have dramatically improved and I picked up about 3-4 MPG doing it, but it is still sluggish over 55.

as a side note I know that its got an exhaust manifold leak, which I am planning to fix as well. Any suggestions?