Is your queue bounded?

Does it reject entries when service times are too high?

Your debugging effort may become more predictable when the system measures the time workers take to complete.

I note you say it used to work overloaded. I would argue it probably was having hidden problems. Perhaps ask those people what the acceptable service time is and lock it in by refusing new entries when it is exceeded.

If they want both infinite queue length and consistently acceptable service times then you must add enough work resource to do that.