when it's been sitting a few days pull the cowl... study the linkage till you find the linkage to the timing advance... without starting the engine have a helper shift into gear and push to full throttle while you watch that linkage closely.... see if it advances fully on it's own and if you can help it along if it does not.....
Years ago I had a chevy suburban w a 350 that was a real dog unless you put it in neutral and held it at wot for at least 15 seconds as it slowly revved from 3000 up to 5000 rpm and then it would run fairly well for the rest of that trip till it was parked for a few hours..... Turned out to be the mechanical advance in the distributor was gummed up and VERY slow tot advance..... half a can of brake clean, some compressed air, and a spritz of wd40 and that old junker ran like a new truck.
COULD be just as simple on your merc.
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