Guidance needed with network performance troubleshooting

The customer has a small basic non-domain LAN in the office where they run an inventory program on 3 computers.  They also have an old PC out in the warehouse that collects inventory data, and once a day an office computer imports the collected data over the LAN.  

In recent days the office sometimes loses its mapped drive out to the PC share in the warehouse, or when it IS connected, the data it imports takes many times longer than in the past.  Under normal conditions it would count 100 records per minute, but now it only brings in 2 or 3 per minute.

They want me to come fix this intermittent problem--but only after they get finished using the warehouse PC in production two weeks from now.  (They are seasonal.)  It seems to me that it may be related to the switch in the office, the fiber run to the warehouse, the switch in the warehouse, the network cable to the old PC, or the old PC itself.  I think they also have some security cameras connected to the switch, so I wonder if one of those could go crazy and cause this intermittent slowdown.

All that said, I need guidance on some diagnostic software or hardware that has a good chance of helping me isolate this intermittent problem.  Understandably the customer does not want to pay for a wholesale swap out of every item I mentioned above--but of course they expect me to resolve it before their next busy season--without the benefit of being able to diagnose and test while they are in production for the next two weeks..  Any suggestions would be appreciated.