What bike is it? It could be carb icing, my zx9 used to ice like a bastid in foggy weather, despite having the heating matrix fitted, something to do with the big airbox. Silkolene pro fst will fix it, or 99.9% pure isopropanol, which is what i used to buy from Ebay in bulk, i'm sure i still have a 5 litre in the shed which is half full, it's yours if you want it.