Hey AH
How far away from the carburator is the tank mounted? In relation to the carb, is the tank on the center line? How far off the DA factory needle setting are you on the carburator?

Almost sounds like you are lean on the low end setting... Slowly decreasing in RPM at a idle till it dies. And possibly a touch rich on high end when you richen the low end up to keep it running, causing the gurgling while flying at higher RPM's. Would be interested to know where the needles are currently set.

Oh, have you checked the small screen in the carb for blockage? If you haven't, you might want to before your next outing.
KJ