I have a Pentair MasterTemp 400, which is about 2 years old. Recently it started cycling off and on while attempting to heat my spa. It cycles every 2-3 minutes and runs for a minute or two at a time. I did run it for about 10 minutes while circulating my pool as well, and didn't detect it cycling at all, which makes me think it may be pressure related, but I'm not sure that to look at next.
Cleaned my filter cartridge today, which helped flow, however it didn't fix the problem.