Idles high while moving only, occasionally sputters & stalls

Some background: car has not been well cared for. Oil changes at 10,000miles, no tune ups, stuff like that. While pulling into my driveway one day 3-4 months ago, the car began to sputter down around 100rpm's or so. This has happened numerous times, and the quick fix has always been to floor it -in neutral- up to 4000 or 5000rpm and then let it settle. Usually back to normal. This time, I decided to let it just keep sputtering and wait and see what happens. After 20seconds or so, it corrected itself, then promptly rose to 4200/4300rpm for about 30-40seconds. I let it settle back to normal idle, then shut it off and went in my house. Since then every time I drive it, it revs itself up to 2000rpm the moment the car begins to move, even I neutral or in gear, but with the clutch depressed, and it won't drop to normal idle until 3-4 seconds after the car has come to a complete stop. Also, I've never had to do any work on the clutch or transmission. Still the original clutch & clutch fluid. Still the original fuel filter too. No service engine soon light either.