The low (suction) side will drop pressure when the compressor is on, the high side will go up.
If the compressor cycles a lot, it could be low on refrigerant, have a bad cycling switch or a restriction in the low side such as a stuck expansion valve.
Depending on the ambient air temp, with the A/C off you should have ROUGHLY 70-90 psi on both the high and low side. With the compressor engaged, the high side will be anywhere from 150-250 again depending ambient and if you are using R12 or R134a which runs at a slightly higher pressure than R12, and the low should drop to maybe 15-20psi and then disengage the clutch and rise back to around 40 psi and the clutch should come back on.
Those pressures are all a general range where they run and shouldn't be taken as exact numbers. Ambient temp, air flow through the condenser, air flow through the evaporator all have an effect on the pressure.
Also, if you are running R134a you want the charge amount to be 80-90% of the R12 capacity called for.