i had my oil cooler go on me.. it seemed like the HG was gone. problems only surfaced when the motor got really hot. i suggest buying one of those tools for those annoying little ring clips... search for ring clip pliers on ebay (i think that is what they're called)
i hope it is your oil cooler. easy fix without a/c, bugger with (unless you want to disconnect your lines, or they're already disconnected)
i think the oil cooler nut is 27mm. drain oil and coolant. pull hoses from oil cooler, take off, put new one on, hook up hoses, install a new mann/bosch oil filter, put some oil in, and coolant. supposedly it is a good idea to flush with a t-stat on these motors. so get er done! once she's all flushed (as per bottled instructions) drain it, and fill up with coolant.
if you have some special tools you can test the oil cooler. i just decided to tear into mine hoping that was my problem. i guess a hand vacuum pump would work. block off one coolant passage, and pull vacuum on the other. pull as much vacuum as you can get and let it sit for a good half hour or so.
i was going to use
www.steelseal.com if it wasn't my oil cooler, as i didn't have much experience or time, or money to dump into the old lump of a box. you really don't know what to expect when pulling a head. except one thing: expect anything!
tools you need: 12mm 12pt tool for head bolts, lots of cleaning supplies. (make sure not to drop things in any of the passages) a good set of tools, a torque wrench, timing tools, a good machine shop to check the head for you. if you know someone who's changed a head before, they could definitely help out! theres probably way more too.
it isn't really as simple as unbolting the head and throwing on a new gasket & bolts. but that and getting it resurfaced, pressure tested, and checked out is definitely how to do it properly.
good luck!