What causes this sporadic short-cycling?

[Eggy]

Hello!  Thanks for your help.

My hot tub often goes into a state where the heater short-cycles - turns-on for a minute or two, then off for five minutes or so, then on for a few minutes, then off for a few, etc, etc.  It happens only occasionally, and last for just an hour or two (as best I can tell).   What could cause that?  A bad temp sensor?  ...dirty/clogged filter??  And, more importantly, what can I do about it?!  At first I thought it might be related to the specific temp setting (I thought it was occurring only when temp was at 101 degrees, and not occuring when temp was 102), or related to outdoor temperature (winter, in Michigan).   But I've kind 'of ruled-out both of those (that is, the short-cycling happens at other temp settings, and over a range of outdoor temperatures).   I *think* the filter is reasonably clean - water flow seems fine - but am willing to replace it if you think that could be a cause.

The tub is a 2005 Viking Eclipse (modest, and old, but I love it! ), with Balboa controller.  It has not had this problem for most of it's lifetime, but last year I swapped-out the failing circuit board & heater element (with a used board & heater).  I may have also swapped the temp sensor - can't recall.  I could swap back to the prior sensor, and even the heater, but would like to gather more knowledge beforehand.  Can you help?!

Thanks Again!

[CanadianSpaTech]

Remove the filter and remove it from the equation see if it still happens. Start there