Try a couple or three things here...

1. Increase the Pause on reply to 200mS from 2mS... and if this works, work down from that...

2. Drop your baud rate to 300 and work up from that... even with a calibrated internal oscillator you may still significantly deviate from 4MHz...

3. Prefix your Transmit string from the slave with say eight nulls... like so...

SEROUT2 gpio.4, 16780, [REP $00\8,"SYNC", Counter]

This will shift-out any lurking line garbage before good data arrives...

You may need to impliment any or all of these options.