Q: Car randomly stalls at idle and at high speeds when clutch is pressed

Last year December I changed the cam belt and put in another one but couldn't tighten to (tension). I then took it to a mechanic and when I went to fetch it it couldn't start and the problem was da distributer but anyway I had it replaced and so that's when da stalling issue started and recently da coil burnt and I had it replaced and still da car was stalling, what this car does is dat at high speeds if you press da clutch paddle or if you put it in neutral da rev counter will go up and down multiple times and then end going all da way down to 0 causing the car to stall, also at normal idle the car just stalls , sometimes it'll stall when da fan goes on or when it goes off. ive taken it to different mechanics for diagnosis but it doesn't show any faults. What could be the cause of this problem ive parked it in da garage for almost a year now .

My car has 84000 miles.
My car has a manual transmission.