Miss fire past half throttle!

I have a 97 mitsu eclipse with 159,000 miles on it. The motor is a 4 cylinder, the 420a non-turbo. I bought the car a month ago. The car is sending a cylinder miss fire 1 and a mult./Random cylinder missfire code. I had a small shop that changed the ecu and the cam position senor 3 weeks ago. The cars issue is that when you go to give the car more than half throttle it will miss fires and wont put up speed. I then had a mitsu dealer look at it and they told me there was water in the tank also so they drained it and etc. Also they told me the ecu wasn't the right one or wasn't working rite. They wanted 1300 bucks to put a new one in soo I got the car back. I then startedmessing around with the car one night and unplugged the coolant temp senor and there was no issue with the miss firing passed half throttle. Iam unclueless on what it could be. Could the ecu need reflashed? Could it be a head gasket going out? I would greatly appreciate any ideas on what this could be. Thank you very much for your time.