I have a 48k Apple ][+ with a Z80 and ROM switch card (with original Apple ][ ROMs installed on the card), however, I do not have a language card at the moment. The computer works for the most part, but every so often, it gives me some graphical glitches. It's usually fine for the first hour or so, but after a while, it would lose horizontal sync when I hit certain keys (mostly the space bar). The effects were temporary at first, but eventually, the picture would cut out completely or come back with random characters on the screen. Either way, the computer would be nonresponsive for about 5 minutes or so, then come back after power cycle.
The keyboard had been problematic from the beginning (shift and other keys would cut out after prolonged use, sometimes the reset key as well), but I couldn't rule out a loose or dirty connection to the PSU (the machine was pretty gross when I got it). When I first got it, I got no picture and no beep. I washed the motherboard and cleaned out about a dozen pine needles from in between the sockets and slots. I also scrubbed out some light corrosion in the area between rows C and D, as well as in slot 0. Replaced multiple ICs (not necessarily damaged, but chip swapping to diagnose a Franklin ACE and fix the no picture issue that might have been the unadjusted potentiometer at J14) and the socket at C2. Long story short, eventually, it came back to life.
So I sprayed some more contact cleaner into the power connector and reflowed all the solder joints on the keyboard. I also fixed some dry sockets on the connector to the encoder board, which seemed to resolve that problem (for now). The 4 and 7 keys are a bit stubborn now, but overall, it's an improvement. But now, every so often it's flashing a Garbage screen when I use CP/M? It doesn't seem to interfere with operation like the previous glitch did, but I'd still like to try and fix it before it becomes a serious problem.
I'd love to find some good diagnostic software. I tried Apple // Diagnostics, but I keep getting an "unable to load ProDOS" error. MECC Computer Inspector works, and I've tried Apple][ RAM test but all they seem capable of telling me is that the RAM is OK. Everything else I've found has been for the //e or //c, but they've also been far more informative with those systems than what I have for the ][+. Any suggestions?
Update: Garbage screen is flashing in BASIC as well. I got a better look at it, and it appears to be the same ? and inverted @ monitor boot screen from the ROM switch card? The thing's off, so it shouldn't be doing that.