One of two things... Either your cable (throttle) is frayed inside the casing, and about to break. Or you have a vacuum leak in the case.
Disconnect the cable from the carb, and twist the grip to see if it is SMOOTH, and NO sticking.
Second you may have a base gasket leak from changing the head gasket. You could pull the cylinder, and change BOTH gaskets to see if that does it. DON'T just put some sealer crap on the old gasket cause it will get into your rings, and cause different problems.
Those are my ideas but I still strongly suggest a Vacuum test, and a compression test.