Check all the "Project TB2" threads.
But to summarise:
Cooling issues with TD
Waterpump shed is vanes (instant overheat)
Rad silted (overheats on long periods of heavy throttle)
Cylinder jets blocked (overheats on long periods of heavy throttle)
Side effects:
If you hit 100C on gauge (which is 110C in reality), you have probably damaged the head and or the HG
You can bugger around with fans and other shit for months, and spend a fortune trying to avoid the 2 slow overheating issues above, but eventually you'll have to accept those. Hopefully before you've done further damage.
I'm guessing that the rad is blocked, or at least partially blocked. I checked it earlier as I pulled up within seconds of the viscous fan kicking in, the centre of the rad was hot, so at least some water is passing through it. Checking it after a few minutes of the viscous howling reveals a rad with stone cold centre and hot edges.
I've only had it up to 100 on the gauge once, very briefly. If it has buggered the HG or head then so be it, it'll have to take its chances until it finally fails. I'm changing the water pump when it arrives, and having another go at cleaning the rad (internally). Cheapest replacement rad I've seen is just shy of £100, and I haven't got that spare for a couple of months at least. With the car only costing me £250 in the first place, I'll take my chances. Unless anyone has a cheap second hand rad I can take off their hands?
If the oil jets are blocked then I'm not really sure what I can do. I changed the oil earlier this week, used some additive to flush the old oil before dropping it all and replacing with GM's finest 10w40 semi-synthetic. The car has history up to 140k (it's on 164k now), and I know the oil was changed at least 12 months ago as by coincidence I got my oil filter from the same place a previous owner bought theirs from, the registration was still on their records. As I have enough oil in stock for another couple of changes, I may drop the oil again soon, just to be doubly sure.
I'm not going to give up on this car just yet! I've 24 years history of running banger money cars on a shoestring budget, and I haven't too many total failures to my name. It still drives spot on, noisy fan excluded, and its such a comfy old barge.