Can you try with couple of variants to try to isolate the cause? Try each one independent of the other.
1) use `modparam("dmq", "num_workers", 1)`
2) use `modparam("dmq", "worker_usleep", 1000)`
3) use `modparam("dialog", "track_cseq_updates", 1)` - this may break authentication done with `uac` module, but I understood you test and can reproduce it in the lab