Hi,
Unfortunately these problems are very difficult to solve, because we have a not clear way to reproduce it.
It may happen only with a specific RC object, only in certain frames or render nodes, or completely random. As other bugs, we need to reproduce it consistently, in order to debug it.
Do you say that caching railclone doesn't help. How did you cache it exactly, using Display->'Proxy Cache' or with RailClone Tools ?
If you have not done it already, please try using Display->'Proxy Cache'->Embedded for all your RC objects, and check if problems continues. It can be enabled easily from the RailClone Lister.
That may give us some clue, about in what step the problem happens exactly.