hi, when i start the engine, the (rpm?) gauge moves up all the way to 2 and when i put it into reverse/drive, the car kinda jumps/bumps. a mechanic says i'm not supposed to change gears right away after i start my car, but i don't do that, i let my car warm up a bit before i change gears and that's what it does. what should i do? thanks

ps- i have a 95 ls