2005 Kia Optima Timing belt and Transmission

Hi, In January 2015 at 118,000 miles my timing belt and crank sensor stopped working while I was driving. The timing belt was missing teeth and cracked in a few spots. I took it to a repair shop and they fixed the timing belt and crank sensor and also added a new surpetine belt. After spending about a grand on it, I got my car back and have had problems with the car not having power. I can press the gas peddle all the way down to the floor and not move very fast or barely anything at all. The rpms would reach about 5,000 and then drop. It is hard of shifting at about 30 mph or sometimes will not shift and I'll have to throw it into the slap shift in order to shift. I have taken it back to the repair shop 3 separate times for the surpetine belt because the 1st time it was too loose, 2nd time it was too tight, 3rd time I was missing a complete rib. They kept saying that it was that belt causing the issue. I took it back again two weeks ago and they looked over the work they did and said that everything they did was fine. They told that when my car stopped that it messed up my transmission. I'm not a mechanic but it doesn't seem like that should happen. I have also taken it other places as well and they said that they must have messed up their work. They said they are continuing to test and see where the problem is but I'm just sure that all the problems with the car started after they fixed it. Please give me some insight. Thank you