Auto transmission issue (intermittent) coolant link?
 

Hi, I’m hoping someone can help me diagnose the issue/issues I’m having with my 06 Grand Voyager 2.8 crd auto.

as said it’s intermittent but usually occurs after driving for over an hour (something I rarely do)

first the temp gauge drops to under 1/4 and loss of heat at the blowers

then the auto gear box will stay in each gear much longer ( to get it into 3rd you have to rev over 3000 and 4th also is much higher to get into than normal
the car will sit at around 50mph with slightly but noticeably higher revs
it doesn’t sound like it wants to go any faster than that really

obviously I stopped as soon as it happened today as I switched the engine off the fans remained on for a couple of seconds something which I have never noticed before

things I noticed at the roadside were that the top hose was cold and soft, as I squeezed it the water in the expansion tank moved
also there was no pressure in the expansion tank I very carefully loosened it but it was cold and zero pressure
the engine bay didn’t seem overly hot and there was no creaking or other cooling down sounds

after about 15 mins I started off again and the issue (with the gearbox) had gone for a few mins but came back there after
i drove home about 20 miles and switched off again fans stayed on for a couple of seconds again and then nothing, no adverse heat, no burning smell, no cooling sounds

so after about an hour of limited research I start the car with the expansion tank cap off ( to burp it???) it warms up in expected time but now the top hose is getting hot and hard, zero bubbling in the expansion tank though

i check the temp in the car, gauge is only just over 1/4 but heaters are blowing hot

There are lots of posts about the cooling system on here, I have replaced the original rad for an aftermarket one and I presume my thermostat is stuck open as temp gauge always sits just below half but none really cover the symptoms I’m getting with the auto box

can anyone explain what’s going on?