Don't want to frighten you but I have many years experience with these engines both regionally racing and bone stock recreation. Souns like you are lightly seizing one cylinder. This. Will manifest itself exactly as you describe. While I have personally seen many different reasons for this, by far the number one reason was a crank that had twisted a few degrees out of index. In effect this advances the timing in that cylinder causing it to overheat and lightly seize. Cools off and starts but will continue to do this until the repeated light seizures reduce compression and begins to be a very noticeable power deficit.