My sprinter engine has suddenly started running rough in the bottom half of the rev range. Worked fine this morning, but when I started it after work this afternoon, it started idling rough. Does anyone have any suggestions as to what it may be? I have already checked the spark leads and distributor. Don't think it will be the plugs, because they were replaced on the weekend. I'm hoping it isn't a blown head gasket, but so far, it is all I can think of. Any help would be greatly appreciated.