Is U20 on the CPU socketed - if so, it might be easiest to replace this first - they often show symptoms just like you are describing. Its a ULN2803 IC.
As for the clock, i've got a tester where i can test each of the opto's individually, but you would need to send the board to me. I've got one spare TZ clock board coming from Ingo, let me know if you are interested. His boards are top quality and use readily available optos (the original ones are near impossible and hideously expensive if you can find them) - plus they can use colour changing LEDs!
