Any other options before I drop $700 on a new Max-E-Therm heater core?

I have a Max-E-Term 400k BTU heater that once it reaches the high 80’s it begins to shut off the burner, throws a check heater light for about a minute then the heater re-lights for a minute or so and the process repeats again and again. I looked under the panel and it is throwing an HLS light when this happens. I have gone through the forums and this is what I have done so far.

I checked the high limit switch with multi-meter and it opens at 135°.
I checked the thermal regulator and it...

Any other options before I drop $700 on a new Max-E-Therm heater core?