Electric fan

Here is a strange one for you all ,this morning on starting my 328 it was running pretty rough give it some peddle sounded like miss fire drove it down road about a mile and still rough idle and misfire so turned round drove back home on leaving the car on my drive I noticed electric fan going even though I had keys in my hand temp not even half way cat not even warm so I unplugged the connector and it went off tried car and was running fine no misfire took it out run sweet as a nut plug fan back in it come back on and was running lumpy again just can't work this out mine as a viscous fan to ,the elec fan on front of rad is the one stuck on anyone any idea got no warning lights on nothing