Ok with all that said , what causes the High limit to trip frequently with the elements checking out ok.
ie with the elements within ohms and drawing the right amps.?
No visual check of the elements.
2 physical circumstances can also cause this:
1. small leak anywhere above t-stat and water running behind t-stat will cool t-stat enough to overheat water in tank.
2. same thing if t-stat is not tight against tank.
also if lower t-stat is not working properly it can overheat water.
but of course in all these cases the water is getting too hot, and kicking reset. If the reset kicks and the water is not getting too hot, then never mind.