Rough acceleration

My 1990 4 cyl/auto convert developed a rough acceleration this week. Last week, after hearing a strange noise from underhood, found a piece of plastic rubbing on the exposed timing belt. The belt is also looser than I think it should be. I do not know if this loosness is from expansion from the rubbing, or if i can crank on the tensioner. A few days after that, after not driving the car for 4 days, I took it to work (usual practice). The car does not have smooth acceleration anymore. It will go smooth for 500 rpms's than jump the next 200 or so in one motion, all the way across the tach. it has smooth idle, does not stall, and feels a little "bucky" from a start. The car held speed and did not buck or jump with the cruise control on at 70 mph. tow questions:
1- can I crank on the tensioner to tighten the belt, os should it be replaced?
2- any ideas on what could be causing buck ride?
thanks
andrew

Update-- only happens with car in drive, no signs of issues in park or neutral. Checked trans fluid, was a little low from amount lost in radiator swap. Added fluid, still chugging.

OK..replacement is needed, however will this cause my chugging and hesitation?

while I appreciate kevinspann's opinions, does anyone support this theory? I am on a limited budget so I do not like to throw money at a problem until it goes away. I know we had a hard time tightening the distributer down before, could it have worked loose and the belt having play in it be a coincidence? My daughter has health issues and I use this car to clear my head with the top down, so it is important to me to do the correct thing. Thanks.