99 ford contour jerks at specific speeds

Car started to jerk when 30mph and up. I changed the fuel filter and oil. Car was ok for a couple of days, however it started to make lound sounds, needing a muffler. i took it to the shop and it needed a flex pipe. All was well for a day, than the car had trouble staying started. It would drive fine, but when you are stationary at a light or stop sign the car wanted to stop. Took it to the shop and the guy said it need a pvc pipe/hose. The guy told me that the pipe had rotted and it was at the back end of the engine. Car was good for several days, however it now started back jerking at 30mph and up. It will not do it all the time but sometime. It’s like the crusie control is going on off. You can step on the gas, but the car will not go any faster, ‘just stay the samespeed.’ When speed is drop it will be ok, but will jerk for a period of time and then stop. Can anyone figure out this problem?

Mine to. I was hoping someone would post info about your problem. I was thinking it is the transmission, I hope this is not the case