B19rr,
Most likely the issue is a lack of DSP resources, IF your system is not being attacked from outside. Most of the time if the system is stable, but you have a lot of phones then the shared resources are your limiting factor. The only way to get more DSP resources is to add a PEC-1 card. The only thing the PS-1 would help with is processing power and the voice processor since it is moved to the PS-1, IT DOES NOT PROVIDE RESOURCES, and since your CPU is at 25% it is not the processing of the calls that is your problem. Also, since your CPU is at 25% it most likely is not an attack from the network, but that could still be a very real concern so I would look at that first.
Thanks,
TE