Burbling in mid-range and quit after throttle back usually imply a rich low needle setting. try to lean it to 1.6 turns out.

Difficult to start after a few flights is an indicator that the ignition battery should be recharged/cycled. It happened to my 5-cell nicad ig. battery. Recycling solves the problem. Also make sure the ignition module's box is not completed wrapped in foam.

--qc