[Supras] 89 NA kicked the bucket

[berniek at technicaldevelop.com]

Coolant appearing suddenly all over the place does not sound like a BHG.  With the miles (er, kilometers) you have on the car, one overlooked item may have been the 90 degree bent water pump bypass hose, impossible to see, as it is in a pocket in the block on the engine side of the power steering pump and behind the timing cover.  Mine went also, and the only way I found it was by pressurizing the radiator with air at less than 13 PSI and listening for leaks.  That was done by drilling out an old radiator cap and soldering in a piece of 1/4" copper tubing connected through a pressure regulator to an air compressor.  The hose looked OK with no pressure, but sure enough, a small slit opened and hissed when pressurized.

If you were not aware of the leak at first, rough running and a drop in oil pressure may have been a result of overheating.  The oil will thin out at high temperature and make the pressure drop.  If it did overheat, you may have shortened the head gasket life.  In my case, the hose went at about 140K miles, and the HG went at 156K.  

Pressurizing the cooling system with air may be a first step in determination of the cause of coolant loss.  You can listen for leaks with a length of hose held to one ear.  

Good luck.