2005 4.3L Gxi V6 starts and idles fine and runs smooth at accelerated speeds without a load. Unfortunately, accelerating under a load, it will climb thru 3500 rpm's for about 10-15 secs before it loses power and begins to backfire. It does seem to be more prevalent when it has sat unused over the the winter months. it slowly clears up over a period of time each year. I stabilize fuel each fall and mix with fresh fuel at first use each spring. Also it is a twin engine cruiser and the other engine has never had this issue. I will add that this engine has had an occasional issue with a bad electrical plug connection with its fuel pump. But it is an all or nothing problem where the engine dies and won't restart until I tighten the connectors in the plug and the pump begins working immediately. Any ideas would be appreciated.