Engine or Power problems

Got the car back from the mechanics shop on April 22nd 2016 after getting the valve cover gasket replaced. Then my car starts acting up losing power and the lights start dimming in and out when the engine stutters. I made sure the oil was full including transmission fluid and everything else. But, whenever I have it in Park ten minutes in it starts to stutter while adding transmission fluid. Whenever I put it in reverse the engine some how loses power and stutters even while I'm driving most of the time the engine has an issue increasing speed slowly then the RPM's would slowly increase while the needle slightly bounces to read it. People are telling me the transmission is going out but, it shifts smoothly and works fine. So, right now I don't know if its the Alternator, Battery, or the Engine having issues. Please help so I know what to ask the people that messed with the Twin Cam 16 Valve Engine