Strange that it's happening right after you shift into second. You don't think that your motor can't handle the low RPMs of the second gear in combo with that oil, do you? That kind of sounds similar to when I was screwing around with 18 tooth front sprockets with mine. Once it hit second (after taking forever to get into it), it was just too much for that motor to pull. Also... it could be combined with you being overjetted. I have almost the same set-up and I've done fine with a Uni filter and a 78-76 jet. What's your gearing?