Can you talk to just the temperature controller without the multimeter on the bus?
This is a long shot, but with two GPIB instruments, you should only have two GPIB cables. I had a test rack with ten instruments having bus problems, turns out there were 14 GPIB cables in the rack.
Another long shot that some times works is to change the order of the instruments on the bus, instead of C-A-B try A-C-B
