I still have my PF-500. A couple of times I had my sound cut out, changed my guitar lead etc etc. (my first one went wrong, fault light came on etc)
Then at a gig is Cardiff I used 2 x cabs. One started cutting out, so I changed my speaker leads from Speakon, to jacks and never had the problem again.
I don't know how a speakon connects but I suspect there is / was a dirty or faulty connection somewhere. Either a cab, or the amp. The fact that one cab cut out and they are daisy chained probably makes it the cab? (I can't remember if I changed the speakon lead for a different speakon lead)