Problem with speed / idle

Hello. For some time now my engine has been playing a bit crazy. Mostly when I step up the car a little bit more that means pulling the engine up to 3,500 turns and then the clutch kicks to switch, my speed remains constant to 3,500 turns, when all this happens then and I give even more gas e.g. to 4,000 turns the still constant remains, I let the clutch come back (when driving) the speed goes down to e.g. 2,500 turns and my car gives gas by itself ( And I don’t have a cruise control ) When I switch off the engine and start again it stops on its own gas. So that can’t be the cable train. I don’t know if that’s the throttle flap or if that’s my LLRV that’s packing around. Especially it’s enough if it’s a little colder that my car e.g. runs out at the traffic light or if the speed is going to drop or if I start it that the speed varies between 450 and 1,500 turns but after a time rests or can it be that the whole problem lies with my spark plugs? Have platinum spark plugs from NGK R, I don’t know more details. Lg, Dennis