I've had my bike inbetween 200 and 500 miles and I had my first two chain slips. Both of which happened when I was going down a steep gradient for the bike. I am thinking that it was most likely because I may not have kept constant tension on the chain causing it to "jump" off track. Is this the case or does it need to have the chain cleaned and lubed? Also how often do I need to clean and change lube on a fixie? I'm new to this type of stuff coming from a hybrid bike.