Engine runs strong for first 15 - 30 minutes from cold start then wants to chuck its cookies until it's shut down and then re-started a few minutes later. Top end's been rebuilt, carb cleaned (new main and pilot jet), changed fuel pump, fuel lines, impulse hose, CDI (new and used), ignition coil, caps, re-vented gas and oil tank though none of this has helped. It's like the ignition system is cutting in and out making the sled surge back and forth. Stop it for a couple of minutes and you're good to go for another 10 - 15 minutes until it starts all over again. It's a good ol' sled when it's working. Any ideas?