concurrency problem in 5.07

Hi, We have an old sun ss1000e with solaris 2.4 and informix 5.07 on it. This system has a strange and unexplainable performance problem: when more than two queries run concurrently, the response time is highly degraded. All of the data that the queries are reading is in informix buffer cache, so there is no i/o time. The inquiries are read only (cursor stability), so not too much locking should be taking place.... So, why can the system not take advantage of the 6 CPUs that it has. response time example: one process 27 seconds two processes 33 seconds (average) three processes 66 seconds (average) four processes 96 seconds (average) It seems that up to two processes can efficiently run concurrently. Beyond that, the system degrades quite seriously. So far as I can tell, there is nothing interesting in 'sar' reports when the system is showing this degraded performance. The overall CPU utilization actually goes down as more processes are run concurrently. Has anyone seen this before ? Is there a know fix ? The only idea we have at this time is adjusting the 'SPINCNT' configuration parameter... Perhaps double it to 600. Any thoughts on this idea? thanks rob