Informix Performance

We are running a system on a Tandem Non Stop UNIX V5.3 machine with Informix OnLine V5.05. Problem : Over a period of time we notice a performance degradation when running a full system performance benchmark application. I also have a simple set of performance benchmarks (eg 1000 inserts into a database table) which I use to determine how well our system is doing. The CPU time for the sqlturbo processing of both our main benchmark application and my simple benchmark program increases quite considerable (eg 100%). The non sqlturbo processing time remains the same. Approach : We have monitored the UNIX system using sar and tbstat, and have not really found any problems in the figures collected ie we do not have a problem with paging, most of our page activity is in cached memory, and we are not running on a large enough database that would be effected by any tuning tweaks mentioned in the Online DBA reference manual. It has been noticed that the number of LRU Writes appears to be higher when the system is running slowly (and LRU writes > Idle Writes), but the cached memory % are high for both writes and reads (>90%). Have also read excellent book by Suto on Informix OnLine Tuning. I think : After a system reboot the sqlturbo processing time was drastically reduced. This suggests to me that there is some problem with OnLine, a possible memory leak which slows the sqlturbo processing over a period of time. So, our only current solution is a reboot. Help please. Thanks in advance.