Welcome to our forum.
If it's the type with the little square control box, flow thru heater, we usually pull the pack and hook it up to a barrel of water on our bench. You just can not work on it, or troubleshoot while it's in the spa.Does sound like a thermostat, or problem with low speed, could also be the 10 minute timer. Let us know how you make out.Could always box it up and ship to us.
A couple of other ideas:You said it is turning off and on rapidly. I think that's what you meant. That can often mean low voltage to the unit. Even if the thing has been fine for years, check this carefully. I have seen MANY 110 volt tubs with a bad connection or undersized wire run for a long time before this problem comes to light. Put a meter on the inlet wires in the box and turn on the jets. The voltage must stay above 100.Also - in case this tub is new to you - there is a ten-minute time delay built in. When you run the jets and then turn them off, the unit is supposed to sit idle for ten minutes before the heat/low speed jets kick in.
Good advice from all. Â With heating issues always check for clogged filters as first step.Always test, do not randomly replace parts...unless you want to throw away money.