At 55k miles, it would probably be a good idea to go ahead and clean the throttle bodies. It's not all that easy to remove the TBs but it's not all that difficult either. Check some of the DIYs on this site and see if you feel comfortable doing it. While you're in there, I'd go ahead and clean the MAFs (super-easy job; all you need is a big Phillips screwdriver and some MAF cleaner).
But that's just a shotgun "fix" - do some data monitoring/logging to help pinpoint the problem.
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