[MA70-3.0GT]

Couple of quick Q's. When you had the gasket replaced did you also get the head skimmed & both it & the block tested for warpage? If the car has ever overheated really badly it may have sent the head too far out of true to be skimmed back (or possibly it's been skimmed that many times it's made the compression ratio too high).

If both the block & head were true & within spec (and I assume you fitted a standard composite HG, metal ones require further finishing work to head and block) then it could possibly be that the head was not torqued down to the correct figure, which is around 70lb as opposed to toyota's incorrect 58lb.

The first thing I would say to do though (provided block tests are coming back as no gasses in the coolant, if there are then it's a definite HG issue. If you haven't had a block test this is first on the list) is to do a search on "Burping" the heater core as this can cause BHG like symptoms due to airlocks which are hard to get rid of without following the correct procedure. From the "boiling" overflow tank though (more likely air being forced past the pressure cap making it bubble & appear to boil) I'd hazard that the HG is leaking combustion gas into the water jacket.

Hope it turns out to be the heater for your wallet's sake, best of luck & let us know how you get on.