Is it the compressor?

Sounds like the pressure relief valve on the compressor is popping off. Maybe an obstruction in the discharge line, or dryer.

Hard to say. It acts normal until then

 

See page 20. After the air builds to 120 and starts to act up turn the vehicle off. What happens then? Listen and check for leaks. What is the purge tank you refer to? Is the exhausting of air on and off from the bottom of the purge valve? Also look at #4 on page 25.

 

I seen some slide valves in the head of Wabco compressors fail and do something like that. After the dryer poops off turn off the engine, unhook the compressor outlet line and start the engine. is it still pumping air? Be sure to check air pressure to the compressor unloader line before just buying a compressor or head.