kn5000 Rhythm Play Problem

Hi All
I have a Technics Kn5000 which has worked well for many years. Lately it has developed a problem which prevents the rhythm section from working. When I turn on the keyboard and press the Start/Stop button, with the Auto Play Chord on, and a Rhythm selected, no sound is activated. When I press the sequencer Play button the Time Signature on the display shows 0/4, and not 4/4 as normal. If I then play Easy Record, the Display Shows;
Measure = 1,
Time sig =0/4,
Memory = 99%.
Then after playing a couple of notes the Measure jumps from,
Measure = 1 to Measure = 999.
However, the sequencer still records a full song even though the display shows Measure = 999 during full playback. I have removed the Main Board a couple of times and everything has worked ok when reinstalled, but, as soon as I turn off the Power Switch, and turn it on again, the problem returns. Sometimes if I remove the Hard Drive the keyboard might work ok for one or two times on power up before it resorts to the same problem. If I get it to work ok on boot up, it will remain working for as long as I keep the power on.
It appears to me that some chip is not Setting or Resetting on boot up due to some issue with stray signals or grounding, but I have no success in resolving the problem. The Main Board has no signs of damage or bad connections.
I would much appreciate any advice/opinion on this troubleshooting challenge as I don't wish to assign this great keyboard to scrap.
Regards
Jomac