97 Ranger 4cyl 2wd A/C problem

During this past winter I noticed that while my truck sat idling, the engine seemed to go through these cycles of rev-up (only about 300rpm) for a short
second and then about a 5-10 second pause and it would surge again. I thought nothing of it. However, now that I need my A/C I notice the following;
A/C Compressor clutch isn't engaging psi on the Low Pressure side of the a/c is ~ 100 Can hear a relay in the power distribution block click just before the engine surges as noted above (have already replaced the WOT A/C Relay). I haven't had the a/c serviced, so I can't see how it would be overfilled to cause the high PSI. Last summer I noticed it wasn't cooling as well as before. Any thoughts? I'm suspecting the compressor.
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