This issue has been covered immensely for both petrol and diesel engines. I've had this problem in the past and found a couple remedies before finding the solution for my 2.5 V6. The first remedy was to have the Fuel Rails cleaned "Terra Cleaned" along with cleaning the contacts on the O2 Sensors --replace if necessary. That remedy last approximately 18 months. The problem flared up again and I again had those services performed but to no avail -- the problem persisted. The solution for me was to have the Throttle Body professionally cleaned -- that was done in March of 2012 and although of recent the Engine/Drive-Train has come one once in a while, it does not go into "limp-mode" - maybe time for a re-clean? Search the site for "limp-mode" - you may want to filter on petrol engine ONLY!