Low performance

So I recently bought this car, the previous owner had only water in reservoir due to it "not being ran" so I filled it with antifreeze the next day. About 3 days later I drove approximately 10 miles in 45° weather, the car seemed kind of sluggish and running hot so I checked the level and it was at the fill spot, so I go to crack the cap off reservoir and it was so hot it blew the cap off, fluid everywhere etc etc, added stop leak, the bars leak brand and ran the vehicle until I got about 15 minutes of drive time to work it in the system. Few days later it started leaking again except this time I am pretty sure it caused my serpentine belt to slip off. The coolant seems to be dripping from somewhere above my oil pan not sure.
So I put the serpentine belt back on and had bought spark plugs and I had a "mechanic over" to assist. During the coil pack/pads installation he mixed up the order they were taking off the engine. My car starts right up, runs okay now, but there is a weird winding sound now and a huge loss in rpm's, weird shifting at times above second gear and a weird vibration under my feet. Any suggestions will definitely help. This is the first car I have bought and absolutely love ! Just want her running top notch.