2000 Blazer LS, about 145k miles, runs fine other than 1 problem. Any time I get to about 45 - 55 mph and about 1000 - 2000 rpm, the tach/rpm bounces up and down like the power is fluctuating when driving at constant speed and not accelerating. This problem does not happen when cruise control is engaged. This was a problem before, but the fuel pump and a few other things were fixed (can not get an answer from the mechanic as to what), and then it occurred again. Possible fuel pump again, fuel filter, or other issue?