My Cruise Control is Messed!!

[89_3SFE]

My cruise control is really weird right now. If I drive anywhere under about 70 and set the cruise control it will decelerate and then it downshifts into 3rd and gets back up to the speed I set it at, but it won't get out of 3rd and into overdrive. I've got an 89 GT. Is it my vaccuum tubes, or what?

I'm also looking for new seals around my doors and for the hatchback trunk, but I can't find them. Is there someone that knows where I can get a whole set of the weatherstrip stuff around my doors and trunk?

Thanks so much for your guys help. You guys know this stuff better than anyone else I know and that's why I'm asking. Thanks guys!!