'97 1.8T, APR Chip. 80k miles. Recent service: Timing Belt ~1 month ago, oil change (Mobil 1) and spark plugs (stock spec Bosch) ~2 weeks ago. Has an external oil leak (around one of the oil filter bracket bolts...was going to fix this weekend). All maintenance performed by myself...I'll tell you about my last experience at Reeves in Tampa some other time

What happened: Coming back from having my chip updated at APR, my oil light comes on when braking from high speed or accelerating. No big deal, I stop and add oil. ~1+ hours later I notice that the A/C has shut off, then I notice the oil and water temps climbing and the water temp light comes on. If I let it sit and cool for a while the temps go down, but jump back up within 5-10 miles.

The facts:
1) The coolant reservoir is full of a thick, bilious green slime.
2) The car runs fine, just very hot (idle normal, acceleration normal, no white smoke from the tailpipe).
3) No coolant or slime on the dipstick, even after sitting overnight.
4) Over the previous 48 hours I had done over 15 hours of moderately high-speed driving (>55 mph), for 3-4 hours at a time.

Solutions? My first guess is the oil cooler (due to the slime), second is the head gasket. Any other ideas? Should I order a new oil cooler as well as water pump and thermostat and prepare myself for a dozen flushes of the cooling system? Any areas I should look for damage from the thick coolant?

My plan at the moment is to get the new oil cooler, install it, and begin flushing the system. I assume that will tell me whether the water pump is shot...