Yeah they are pretty good if looked after, but they do get slow and jerky to change when the oil gets old, as the oil gets hot it thins right down, sounds like your oil is breaking down with contaminates as it warms up, which is why it is ok when it's cold.

It might not be the cause but I certainly would get it done just to rule it out more than anything as it's what most garages suggest you do first anyway.