2002 Chevy Malibu Car engine jerks when gear is changed to

Hi, I have problem with my 2002 Malibu and hope that you could help me solve the problem.

When I change the gear to D or R, the engine jerks. When I start to drive, it jerks when it changes the gear at low speed. I tried to fix it before by moving a sensor near the bottom-left side of the engine. After I adjust the sensor, it will not jerk. But it will come again the next day, so I have to adjust it again.

Until I drove it in a long distance, it got overheated because of a water tube leak. We replaced the rubber tube and replaced the cylinder heads.

Now, more problems occured after the overheat. 1. The fan does not work anymore. It works only when the a/c is turned on, but only the slow fan. We already replaced one sensor because we thought that it is causing the fan not to function.
2. The engine jerk is not removed anymore by moving the sensors.
3. There is a squeaking sound in the tensioner.

What should I do to fix these problems?

Thanks in advance.