Maybe we should first understand what you mean by "chain-slip".

If you have not changed the chain or the sprockets, it is difficult to understand what would slip unless the teeth of the sprocket are worn to almost nothing.

Usually you don't notice the damage from stretched chain to sprocket "etching" until you put on a new chain and the sprocket teeth don't match it well.

What do you mean by chain slip? Do you mean you put it into gear and the chain keeps jumping off the sprocket teeth and lurching forward (usually with a violent and worry-some teeth grinding sound).?