Usually the PLC should be able to respond to communication even if it is quite busy unless it is performing near its limit such as driving the two stepper channel at 10KHz.
Are you using the Auto485 adapter for on-line monitoring or a third party RS485 converter?
Can you try transfering a simple program (e.g. blank program) to the PLC and then perform online monitoring and do you encounter the same problem of frequent lost communication? If you do encounter the same problem then it could be the PC's serial port, RS485 converter or even the RS485 driver IC on the PLC (75176) that is having compatibility or functional problem.
If you don't encounter lost communication while monitoring simple or blank program, then examine if any part of your program could be writing out to the RS485 port (e.g. PRINT #3, NETCMD$, OUTCOMM, READMODBUS or WRITEMODBUS). ?