OK, since you don’t have profiling available, perhaps check on process size (e.g. RSS) to see if this is increasing over time. It could be that memory is growing / becoming more fragmented and this could contribute.
You might also simply consider upgrading from 6.5.0 GA to a current release to benefit from the updates we’ve made over the past years. This might short-cut any investigation (as if a known bug we’d have to direct you to upgrade anyway).
Sorry I don’t have anything more precise for you.
HTH.