My craftsman 5.5HP air compressor keeps tripping the breaker when it cycles on. not every time, but 1 out of 3 or so. If it doesn't trip when it kicks on, it will be ok until the beginning of the next cycle.

It's on a 20A 115V circuit by itself. The 12GA run to the sub panel is only about 20' of wire. I've tried a different breaker, and it does the same thing. It's a tandem (two breakers on one 115V pole), but it did it with a regular breaker, too. The subpanel is Square D homeline. Nothing else gives me any problems, either at the subpanel or the main. I've even run my MIG welder off the same receptacle (not at the same time) and it didn't trip it.

Any suggestions?