Rpms go up and down while i am driving and when i first start the truck, what is the reason for this ...thank you..

there is not info to be able to give a definite answer
to work it out
you have top know the diff ratio, wheel tyre circumference to be able to work from rpms of the engine to road speed
However as a pointer, most cars will be around 3200 rpms at 60mph as this will be direct drive and od causes a drop in rpms
Being an automatic find yourself a good section of low traffic road and start from stop slowly increase the rpms and watch the rpms. keeping the accelerator steady you will see when the auto changes by a drop in rpms. slowly increase road sped until you see the next drop in rpms and so on. Over drive normally comes in around 55 mph or around 3000rpms
I would think that it is working because 3500rpms at 75mph is reasonable rpms to be expected. If it wasn't working I would expect the rpms to be around 3800 to 4000.at 75mph

After 14 years I think it would be reasonable to have bad catalytic converter. Is there some reason your not accepting this?

See if cleaning the throttle body helps -

TUTORIAL HOW TO clean Vortec Throttle body 5 7L Chevy Gmc

would change top hose plus make sure there is a thermostat fitted

your cable could be bad to your odom.

Hi!
The main reason for RPM going up and down is due to worn out clutch plates.
But firstly check the clutch Pressure plate (diaphragm).

Regards
Berry

try disconnecting the battery for 5 minutes or so, press the brake pedal with the battery disconnected. after 5 minutes reconnect the battery and start the car, DO NOT TOUCH THE THROTTLE/ gas pedal for about 5 minutes. perhaps this will recalibrate the computer. good luck, hope this helps

BUICK RENDEZVOUS IDLES UP REALLY HIGH...WITHOUT PRESSING GAS. WHAT IS THIS?

milky oil means blew a head gasket check for oil in the antifreze