Sleeping scan threads

Hello, Looking for some general reasons on what would cause scan threads to sleep for lengthy amounts of time when there's no bottleneck on disk, memory or CPU. HP V2250, IDS 7.30.FC3, HP-UX 11, 5 CPUs, 8GB RAM (5 allocated to IDS), RAID-5 devices that are allegedly well-tuned for read operations. Engine is tuned for DSS, tables are fragmented by expression (both elimination and parallelism are being achieved), KAIO is enabled, light scans are occurring, processor affinity is enabled for all CPUs, no indexes/all sequential scans. TIA, Chuck