Gears get stuck when in traffic for too long

Good day, I recently changed the clutch kit and my car was just never the same again. The first thing I noticed was the sound I could hear when I move up a steep from first gear, sounds like an old wheelbarrow. Then the car shakes a lot when I go over eighty km/h. The most disturbing by far is this random thing that would just happen whenever. When I was stuck in traffic my car starts to rev high, then soon the car will not go into gear. I have to switch off the car for about ten minutes while pumping the clutch. When I am still stuck in the traffic I have to switch on and off my car because it will not stay in first gear and it is the only way to change gears. It shakes uncontrollably and the slightest lift off the brakes makes the car jump. Once I have rested the car for a few minutes it will go back to being fine until the next time when it happens again. I have had this happen about four or five times in the past month and a half. I have returned it to the garage so many times and when it is there, it runs fine and they cannot find any problems. I even asked them to ride it for a week and they just cleaned the engine and gave it back. This is frustrating because it happens to me when I am so far and alone and so randomly that I cannot say what circumstances causes the revving and gears to stop working. Please help me I cannot live like this!