My Merc Will dirve ok most of the time but ocassonaly it will idle at around 1500-2000 rpm. After a short drive 10-15 mins it will mostly correct itself, but i would be most interested to know what the possible cause is any infromation would be very welcome.

Regards Mark.

Is it automatic transmission??? If yes, check the AT Fluid level, make sure no overfill..... If the AT fluid level is normal. Then, may be the vaccum modulator valve problem.....

Forget to ask, does your idle problem due to not upshift??