Slow Pumps

This week has been my first encounter with a Gledhill product a Gulfstream 2000.
All the imformation I have read so for on this forum has been top notch outlining all the symptoms of over heat.

Heres the question? The manual over heat keeps popping out. the auto air vent is spot on, pump is running!, condense is clear all usual checks.

Q Can the primary pump be running slow and cause overheat.
Q How can the output of the pump be measured (flow + return temperatures not yet recorded i.e 15 deg diff max)
Q Would a faulty pop out switch be the cause and if so how is it tested
Q What is the resistance of the dry fire stat at room temp

I know as you have all advised it is probably a bad gas to water exchanger fault but will a slow pump also show the same overheat symptoms
I have to put these thoughts to bed first. Or something else i can eliminate.
The boiler is around 6 years old it is all underfloor heating and it also is linked to solar panels system with the usual weeping pump valves.

Appreciate your knowledge on this one Thanks Jason