Sounds like your chain is shot. 600 klm is way too frequent. Your best course as already mentioned is replace both sprockets and the chain. Chain oil will attract and hold grit which accelerates wear, so if you oil, you need to frequently clean your chain and not with a solvent for that will wash out the grease held internally behind the o/x ring seal. I use a spray on, dry lube designed for gun actions and so far I have got my moneys worth from my chains and sprockets. I also use one of those chain brushes that cost about $6 on EBay.
