hi guys.

i am new to this forum, but i have used many of the gmc and dodge forums before (i have a bunch of gmc pickups and dodge vans for work). so thats why i am taking up your valuable time and resources.
one of my guys has just bought an old 1991 jetta diesel to get back and forth to work in, and he's having a major overheating problem. the car runs great, but it overheats erratically. it doesn't seem to be predictable, but definitely overheats more when its hot out (we live in toronto, ontario, canada). basically, he drives it and it starts overheating immediately (usually - some days it doesn't do it at all). we have replaced the thermostat and the overflow tank cap, to no avail. the upper rad hose seems hot and the lower one cooler, both with good pressure. the water pump seems to be working fine. the cooling system will hold pressure well (when you check the next day there is still pressure in the rad. i have read numerous postings indicating it could be the rad, waterpump, heater core, multiple plastic ports or fittings, etc. this is pretty ambiguous though. vws are pretty common here, but nobody seems to no how to fix them (probably a good testimonial to the longevity of the vw) does anyone have any kind of simple first-step diagnostic advice that we should do (you know- the kind of thing an experience vw tech would do absolutely first to rule out something)

thanks

sean