Mini Me runs rough at normal temp

Afer i did the mini me (z6 head a6 block) i noticed that the car really runs like shit at normal warm up temperature. For instance in the morning when i start it up its cold and it feels torquey and got some good power but then once it gets at normal temp which is a lil under half way it runs like hell. Now when i did the swap the thermostat was froze shut and i havent replaced it so im running without a stat.. also i have the fan wired up to go on as soon as the car starts due to this problem and im running plain z6 bosch plugs. and yesterday it overheated outta nowhere and the coolant resevoir was over flowing. i dont understand and i was hoping i could get a lil help on what i should do now. car only overheated once due to sittin in traffic but still it shouldnt have done that. im also running straight water until the weather conditions get colder. thanks for the help

I would reinstal a thermostat... Other than that, I can't tell you why it runs crappy after warm up...