A motor will be under loaded in low speed so yes, it could possibly trip the thermal limiter only on high speed while not on low speed.

But a couple question to narrow down the problem.

Motor model#?

Is there a centrifugal switch in the motor?

Is there a single capacitor OR are there two capacitors, one on motor body (i.e. hump) and one in the end cap?

Have you tried to spin the impeller by hand and does it feel rough?