I've been having some kind of problem with my Comcast DVR ever since it was given to me. I haven't noticed anything new the past few days with the fast forwarding or rewind. I know sometimes when I rewind it'll jump back like 10 minutes instantly or the time bar will be all jacked up, and sometimes I set a recording for early morning (like 2 am) for an hour show and it'll still be recording when I get up in the morning. It also doesn't handle super long recordings very well like the olympics. Weird things happen then.
I think the idea is taht you fast forward through commercials and if you hit play when you see the show, it backs it a little so you don't miss any of the show. But I mastered the old fast forward and rewind and this new system is screwing me all up. Still not used to it weeks later . . . and I spend a lot of time with that remote in hand.