I had a similar problem with a 203-6 which turned out to be caused by the unblanking pulse not being set properly. It just needed adjustment. There's a procedure in one of the 203 series manuals which says to use another scope with a 10X probe connected to a -1900V line via a 10nF 2kV capacitor and then adjust a pot to get the correct waveform. I chickened out of the scope part and just adjusted the pot until the trace came right, then went wrong again, and positioned it in the middle of the range where it worked.
Of course, it may be a faulty component rather than needing adjustment, but it looks like an unblanking problem.