Only kicks for a few seconds, then off, and repeats. System is full, no leaks found, all control switches working. Have not yet checked the high side, low side reads 40 when the compressor is on, 18 when off.

Compressor? Blockage?

 

How do you KNOW this? Was it charged by weight accurately? if not, it's just conjecture that is correctly charged.

Your symptoms are of either an inadequate charge (by far, the most common) or the system is cycling due to over-pressure on the high side.

Your numbers don't make any sense. Get a proper set of gauges so you can see both sides and watch the needles as the compressor cycles.