high cached_page_count - when to start to worry and what can I do about it?
The fact that these tables both cause performance problems and use lots of pages in the cache isn't at all surprising. 

I would rather find it expected. You call a couple of tables that (probably) both are on the larger side, and you also do lots of activity on those tables.

Perhaps you have lots and lots of inserts, which quickly causes the clustered index to become fragmented (hence freeing up space after rebuild)? If that is the case, then perhaps the key for the clustered index might have been badly chosen.

I would start by collecting more information. Possibly running a trace to see what activity you have going on.