Comcast audio-video out of sync

I have never had this problem in past with comcast "old" boxes(I have On Demand) but when I had another cable problem recently, comcast installed their newest digital boxes which I assume are intended best to work with hdtv. My tvs are not hdtv and now there is a sync problem with audio and video. Before I call comcast, would it be better to have them put the old model boxes back in to solve the problem or is this unsolvable? They also ran new cable to the house when they put the new boxes in and put in a new splitter and amplifier on the line because of a weak signal coming to the house. Help!