Can an electrical fan issue cause a sudden loss of rpm while driving?

What is your question?

I test drove a 2006 S80 2.5t and while accelerating from a red light, the engine dropped from around 3k rpm to almost nothing. It came back up quickly, but I'm leery about buying this car. The dealership said it was caused by an issue with the electrical cooling fan, but they didn't give me the exact code from the OBD. The CEL was on before the test drive as well, but the salesman said he thought it was because someone probably put regular unleaded in the tank instead of premium. Any help is appreciated!