if it hasn't been serviced by a merc garage and gone over the recommended mileage/service interval, it will go into 'crawl' or 'limp' mode. take to merc and cross their palm with silver and they will reset the ecu chip. This is not 100% the answer but worth checking out before you go trying to mend things that aren't actually broke.
hope this helps.
smartrs, October 2010
