excess oil

Hello everyone. My 91 C2 had a 45k service done 2 weeks ago, including valve adjustments. Yesterday, the car started doing something where it almost stalls when the gas is let off at a stop (in nuetral), then suddenly catches itself and returns to normal idle. Also, the check engine light stayed on for about a mile or so today, during which the oil temp kept rising a beyond normal. When I stopped, the car almost stalled, then the light went off, the temp went down and it ran fine. Then I noticed that the oil level gage seemed high. I checked the oil stick and the level was above the high mark. I don't think it was like this before today. I also noticed a little oil on my garage floor, and this car has never leaked before. What might cause the oil to be suddenly high and leaking? I will take it back to the shop for correction, but any thoughts re what I should be sure they check or do would be appreciated. Thank you.