boston bimmer

so I was driving yesterday and my alt/fan belt broke in my m42 so I got a new belt and put it on but now it will not run right. It will idle a little rough but not too bad but when driven it lags badly up to 2500rpm then accelerates like normal. When it goes back to idle it stalls. The check engine light goes on and off intermittently and if held steady at any rpm it will run rough. My initial thought is that the alternator is bad on a side note I just put in an electric fan and was also wondering how to disengage the clutch fan (snip a wire somewhere or something) or if that could be a problem too (the new fan was installed after the belt broke I was wondering if it might be the fan clutch having issues) please let me know what you think thanks!