Sorry if I missed it, but I see that you've replaced the temp gauge a few times...but have you replaced the thermostat, in the engine bay? That's what was doing mine in, although the problem wasn't as drastic as yours.

Before giving up completely I'd go through and either check or replace all of the hoses. Could be that something is clogged somewhere.

 

it has everything i can think of new radiator, thermostat, waterpump, hoses, been flushed, ran without the thermostat , put back in no change runs fine for a while and then it just slams up !!