On a 2001 chevy silverado 2500HD the ac blows cool in idle or slow speeds but while driving the air gets warm and then cool warm and then cool etc. i cant tell if the compressor is cutting on and off

I'm going to assume you have the manual control version. The relay in the resistor allows the fan to run on high and bypass the resistor circuits. There is a 40amp blower fuse for high speed and a 30amp fuse for the other speeds. The motor is grounded thru the resistor.
Do you have power and ground at the motor on any of the settings ? ?

check if the fuel filter needs replacing

check rear u-joints, also check rear dust shields for contact against drums/rotors.

it sounds like your idle speed sensor it has to be scanned to find out for sure

low charge or too high of a charge.
the low side should read 25 to45 psi at idle.
But, if the a/c behaves this way, there might be a restriction in the orifice tube near the firewall

Good luck to you in fixing this problem.

There is a possibility that the A/C compressor clutch burned up or got disconnected. Try investigating the wiring for the clutch.

There is a fan control module mounted in the air plenum that is known for going bad.Check the connector where it plug's in under the case below the glove box for a burnt connection.Usually the connectors will melt but it can still be bad where it goe's into case.It need's airflow to cool the resistor,s for the differant blower speeds.

you havent check the pressior of the compressor make sure is full loaded

It sounds like you over charged the system, the low side should be 40psi to 50psi with the A/C on high and the high pressure side should read 150psi to 200psi. Anything different, you need to get the system service and try not to use your A/C to prevent the compressor from any damages. Good luck and hope this helps.

In most cases it is the fuel filter. I don't know when it was changed before, but it should be changed at least every 8-10 k miles. Make sure you change the air filter too.