1998 Oldsmobile Bravada Bucking under load

Engine Performance problem
1998 Oldsmobile Bravada 6 cyl Automatic 75,000 miles

In 2008 the mass air sensor was replaced. This was done 4 times before the check engine light would stay off. In 2009 after replacing the head gasket the engine siezed at 65,000 miles and was replaced with a Jasper engine of the same size. In 2010 the check engine light would come on for several days and then go out. At highway speeds after 1/2 hour of driving the car will buck and shutter with a loss of power. When checked there were no stored codes. After driving the car several hours, the mechanic got the check light to come on and replaced the throttle posistion sensor and the E. G. R. Control valve solenoid. Car still bucks and has loss of power intermittently after 1/2 +- hours of driving at speed (65 - 70 mph). This condition stops when speed is decreased and we're off the interstate.