First if this is a 5000lb capicaty truck as stated then it should only have a 4 cylinder engine, Toyota did not put 6 cyl in any trucks smaller than a 8000lb capicaty truck (ie 6fgu40). That stated it can either have the gm3.0 or the toyota 4y depending on the year of manufacture. Regardless of engine as Kevin stated I would start with the simple things like a tune up. From there I would check the governor, then the distributor gear for wear, then the timing belt/chain(depending on engine) for slipping or wear. I have seen all of these items cause issues like this. I would also check the transmission pressures as a problem in trans may be causing excessive strain on the motor. hope this helps.