When mine went it was definitely the Oil Pump. That is also believe to be the cause of most, if not all FMS failures, due to increased pressure against the seal from a worn pump. The argument, which I think came mainly from the states, that it was the Crank Interface was largely discounted as being possible but very unlikely.
This problem with pumps had been identified already, which is why pumps with modified drain holes are around (it allows more excess pressure to escape to avoid stressing the FMS). CW said at the time that the Supra pump has a very small drain hole.
ETA - More reading here where this was discussed at length: http://www.mkivsupra.net/vbb/showthread.php?t=108524&highlight=engine
To summarise all 22!? pages there my engine was sorted out by its new owner by replacing the pump with a good OEM one after just changing the seal didn't work for me. If I were you I would check your pump is worn (just in case) and then replace it with a genuine OEM one, along with the FMS. The only question I would ask is what caused your pump to wear in the first place. Mine was due to crap getting into the engine from some head work (I believe), but it might be worth checking while you've got access to the engine.