Hi there! So I got a new timing belt and new serpentine belts as well as a section of my exhaust patched and the car runs great. Except now when I go above 65 and ease off of the gas pedal the car shudders violently back and forth. If I take my foot off of the accelerator quickly it doesn't do it and it is OK to a certain point but once you hit that sweet spot where you're barely on the gas it does it. It seems like when I first start driving it doesn't do it but after 15 minutes or so of highway it will do it consistently. Also it seems to not do it on flat ground but going down hills I really notice it. My mechanic doesn't seem to know what could be causing the problem.