Automatically out of the box the WebLogic Plugin should demostrate a "sticky" behavior so this is somewhat odd. Do all your Apache Servers point to all available WebLogic instances (ie. a sever wasn't left out in one of the lists)? Do you see any
pattern to this? Are you losing the JSESSION cookie on the client? Is possibly a misinterpretation and the user's session is expired and therefore this behavior is expected?
Off-hand I don't really have a good response for this...