Ya know, we just had a similar problem on a Toyota 4Y. Found a few things wrong.
1. The last time the head was refurbed the machine shop did not tip the valves so the exhaust was a bit longer than the intake.
2. The valve springs were wore out not allowing the valves to return to rest position when the engine was hot.
Engine has about 11K hours on it. Just some thoughts for ya.
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