Well, there's a good chance your rad is partly clogged. I believe this is what's happening with my '83 Blupra, although you seem to have a worse case then I do. I too first thought it was a BHG, but there's no oil in the coolant, no anti-freeze in the oil, the spark plugs have no white residue and there is no white residue on the exhaust tip either. I've flushed the system, changed the thermostat, had it pressure tested and even power flushed the entire system at a rad shop, yet it still overheats. So why do I think it's the rad? Because when I let the car warm up there's a section of the rad where I can place my hand on and it never feels heat. The rest of the rad might be piping hot, but one section, about 1/4 of the rad, always stays cool.
So I'm convinced it's the rad, and it could also be the same for you. You could try taking it to a rad shop and get them to do a reverse power flush on your rad itself (not the entire system). I would do that too, but don't have any cash right now to spend as I'm kinda broke (even though it's probably dirt cheap to do). Instead what I'm gonna do is take my rad to a buddy's shop who has an ultra-sound blind cleaner, who tells me it should be powerful enough to clean my rad (he's used it to clean AC condensers). If it works I'll post back my results.
Good luck!
- Mike
