Something is corrupting the CANbus. You think this happens when indicators used, so I'd go with either the SCM module (steering column, ie connected to the indicator stalks), or the front SAM (connected to the indicators).
I'd edge at slightly the front SAM. They probably need to be coded for what options are on the car, and there are different ones for different ages.
toolman1954 advice is interesting, we've seen lots of cars with some degree of mouse damage and front SAM is in engine bay (its part of fusebox on driver's side), its well sealed and wiring drops down into the car, or is obvious where it leaves the box in the engine-bay so should be easy to inspect. Although I don't think mouse damage would cause such an intermittent fault...
Cheers
Richard