Sorry, no handbook, Gerund, but surely if you are having problems with both transponders, the problem should be relatively easy to faultfind?
Have you replaced the ATC antenna relays? Have you changed the Control Panel? Perhaps other systems in the same frequency range are interfering with with your ATC? (DME, TCAS) Perhaps a particular system is putting out too many suppression pulses? (Pull these boxes out of the racks to see if this makes a difference)
Have you checked the coax connections on the relays? I can't recall if the relays are solid state, but I suppose you could give them a "technical tap" anyway?
Unfortunately, there's no guarantee that this DIAG 42 will be the cause of your problem.
Cheers.
Q.