zeditis

Maybe. Confusion on the higher rateds needing inspecting after repair ?

Pan of boiling water and a thermometer

Normal temps are I believe around 90-95 . I'm guessing that's a aftermarket sensor?

All the lower pressure cap will do is lower the boiling point of the water and seeing as it's only 70 its not the answer to your problem. The more likely culprit is a faulty thermostat (I've had faulty new ones before now) or as already mentioned possibly air locked the heater usually improves a little with higher revs.

I'd check the bolts holding the cover are tight. They've come loose on my own. Then try the stops on the cover the roof is unlikely to have moved further back if it had it wouldn't close properly.