My father has an old Eaton Viking 36 frost guard dehumidifier from the 70's.

When turned on the compressor will cycle on for 3 seconds and turn off then cycle on again after about 20 seconds for another 3 seconds,etc.
Filter is clean and the fan runs with no issue.
Took off the front cover and it's not dirty inside.

I took a temp sensor off the coils on the backside and nothing changed.
After putting the sensor back on the coil it ran perfect for several hours with the compressor always engaged.
It would eventually shut off and restart later.
Coils were colder and more water was dripping off them so I don't think it's low on refrigerant

Then it started the 3 second compressor cycling on/off again.
I can only see one sensor in the back.
What's the likely issue and are parts available?