I have a 1997 gti with low hours that I bought 12 years ago. It's been winterized every year and stabil put in the gas. First time out this year and it ran fine for about a hour and then it started to surge at steady throttle speed. Not real bad but noticable. Just woundering what could be the problem? It had a 1/2 tank of gas and I tried the reserved setting on the fuel valve and it still surged. Any help will be great.