I have 08 Nightster less than 3k on it.

I was driving down the freeway going about 70 mph then all of a sudden my speedometer droped to 50, I felt then engine trying to push harder as if I down shifted. I got off the free and this "slipping" happend again the check engine, oil and batt. light whent on then off then on at one point the key light went on. I felt the engine working really hard and smelled it too.

Can anyone tell me what this might be? I just had a new batt put in and I took off my air filter cover... can this be why?

 

Yes, its your air filter cover. It probably came loose and is catching too much wind and slowing you down.