2002 Ford Taurus engine missing

I had the air filter and gas filter changed during inspection. Ever since, about 3 months now, the engine has intermitent miss-firings and occasional rough idling. This occurs at start-up and when stepping down to accelerate an incline. It ialso worsens if heater or defroster is on. After driving for a time , the engine iseems to run fine. No check engine light has come on. The station I go to could not find anything wrong, even on the diagnostic computer and it did not do it while they had it. It is a 24 V overheard cam. I have put a fuel system cleaner in twice since then. It got better but still does it,especially now the weather is warmer.
Thanks.