Shared memory hogs

Hi. Ever since upgrading to 7.30.UC6 on my HP9000 (HPUX 10.20) I find that on occasion one instance will, on occasion, begin allocating shared memory segments until it runs out of all available memory. At that point I get the expected "could not allocate" errors in the message log. Nothing, including applications, general usage, onconfig, OS, or hardware configuration, has changed except for the version of Informix. (And no, going back is not an option at this point.) After experiencing this on at least one occasion, onmode -F failed to free them even when there were no users other than Informix on the system. Most often the command works OK. Tech support tells me that there is nothing in the problem tracking database, and certainly nothing in the bugs that would account for this behaviour. Has anybody else seen this, or (better still) seen this, found a resolution, and been able to move forward? This is so intermittant that it is hard to pin to a causal event, but it is a production database that I can ill afford to be taking from the users on a regular basis. Also, this is but one instance (of this version of Informix) of three on the same computer, and the other two do NOT experience this type of problem. Thanks, David