Don't know the innards of this particular scope, so can't help you in detail.
Your problem looks like a intermediately bad internal connection, maybe warm up of the scope influences it. So check for any kind of board-to-board connections, re-seat them. If none applicable or cleaning the connectors and re-seating doesn't help, check for supply voltages to be correct, and search for bad solder joints. If documentation is helpful in defining and locating circuit blocks, I'd search for the failure somewhere on the data busses from the Main CPU to the DSP part (as I said, I cannot help you in details, you'll have to find this out for yourself or wait for others to join this thread).
Don't know if this scope suffers from that, but check for leaking SMT electrolytic caps - if there are any, remove, thoroughly clean the board and put new ones in.