Yes, 25:1 is probably more oil than needed but the engine should still run OK at that mixture. Yes too, a gas engine reacts to needle settings just like a glo engine does. When rich it has that distinct 4 stroking, not firing every time sound which smoothes out as the needle is turned leaner. When too lean though a gas engine will once again start to miss and run rough where a glo engine will tend to just sag.

Is your engine a new one or one that has some time on it? In other words, does it have any history of running OK in the past or is it an unknown? How old is the engine? If timing is too far advanced and some DLE30's had this problem in the past, it can run rough at high speeds regardless of how the HS needle is set. A wide plug gap can minimize this effect but not eliminate it. Give us a bit more to go on and maybe a course of action can come from that.