Car won't accelerate properly

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After 2003 Subaru Legacy Outback hasn't been driven a few days, it starts normally. I drive it for two or three miles in city driving, then park it on a hill -- or flat -- surface for 15-20 minutes. When I start it again, it SOMETIMES has trouble starting and when I put into drive, it tries to die and moves very slowly, never more than about 5 mph, and I have to touch the accelerator to keep from killing the motor. After it hasn't been driven for a day or two, it drives normally. Mechanics can't find anything wrong. All systems check out OK. Test drives under above conditions didn't produce the problem. They checked out the exhaust system (it had been rear-ended: I was stopped but driver behind started forward before light changed) about two months previously - no immediate driving difficulties. Is this a fuel system issue like fuel air mix? Maybe something got dislodged? What else should be checked? Thanks!!!

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