School bus starts at the touch of the key, my problem is when on the road the first three gears the rpm's will go to 2500 and shift, when we get to forth gear we don't get over 2100 rpm's. Then quite often the rpm and speedometer will go to 0 and then slowly it will decrease speed down to 15mph. We then have shut off the key switch at full speed and in neutral , then turn it back on, guages sometimes come back to full reading but we still have limited power, sometimes the rpm guage will be the only one that returns to normal and the speedometer will stay at 15mph but the bus will do 2500rpm's and 65mph, once in a while the speedometer will jump to run speed or the next time we slow down to approx. 15 mph the speedometer will then increase with speed. We have thrown alot of money at this problem already by replacing cam switch, pedal throttle, idm, found exhaust pressure tube to sensor plugged(new tube), replaced sensor, found fuel pressure at 34psi, replaced lift pump and seals and one way check valves in both cylinders and installed new fuel pressure regulator and new rubber lines. We also installed a new exhaust muffler thinking it might be plugged. Took to Center City International to have a computer hooked up, had a hard time communicating to vpm, tech jumpered vpm power to ecm power and got communication and saw there was no active codes but still the above continues to occur. At wits end need new ideas!!!