OK, ive had enough of trying to work this out for myself. Basically my temp gauge gets to a certain point, around 1/2 (70 deg), then stops. Ive done bench tests, heating up oil, and at around 150 deg it jumps up to H. When cooling though it comes down at a uniform rate.
Ive tried new sender, 2nd cluster set, run earth wires to sender, bypassed sender wire and run own wire to cluster.
I am starting to think that it's a cluster problem (temp sender wire passes through some electronics in cluster before going to gauge) that happens after a certain time, maybe it is common but people don't realise because their gauge always sits at normal. I only found out because I have to heat up coolant to set my thermofan computer, my ECUTALK display (coolant sensor) increase in temp yet gauge doesnt.