I have an OPS 60 and it was doing some of the similar things. I kept messing with the fuel mixture and so forth getting no where. Than I remembered an old OS that was doing the same kind of thing and te problem with that motor was the cylinder lining coming off. So I went to take the head off the OPS to check the cylinder and the head bolts were loose. Checked the cylinder anyways then and it was good so I put it back together and the engine runs like a top now. The moral of the story is, If it acts like fuel but theres nothing wrong with the fuel check something else.