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. 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.