Van Overheating

Right.

The van is overheating at times...
The new air filter means it takes longer to overheat though.

When the engine starts to warm up is OK, but once its up to operating temp (and the thermostat is meant to open) the temperature goes right up near the H mark, but if you accelerate, the temp drops down to a 1/4~1/3 again. Then when you come off the throttle (down a hill for example) the temp goes high again.

I wonder if the thermostat is malfunctioning.
It seems that once the engine is accelerating, there is enough flow from the water pump to push the thermostat open.

Perhaps the spring has failed?

I will go and take the thermostat out and see if it cures it....

Mine does that a bit but not as bad as yours I have had the stat out and tested it to check it opens but I was wondering if it is opening fully from shut at the full temp and as soon as the cooler watewr from the rad reaches it it shuts off totally and then repeating I've just got back from halfrauds but they don't list our vans only the 80's one.

I poured some K seal into the system and topped it up, and it seems to be behaving itself a bit

Still got very hot when going up a track, but once on the road, it soons cools down to 1/2...

Will see how it goes...