I recently got a nice fully functional (thank god) JP 4..
A couple days later i switched it on and noticed that one note was out of tune (more than a semitone!)
At times it would play fine,
It might have been suggested elsewhere - but i managed to track down the cause in 2 mins...
The Voice Allocation board (sits in front of the 4 voice cards) has 2 multipin cables that connect to it (white connectors, rainbow colored leads). I simply tapped the left one and found that it is intermittent (moving it brought the synth back in tune)
I have also noticed that the switches that select the bender moluation (vcf, vca, vco) can change the tuning if the contacts are dirty, as do a few of the rotary dials - basically, there are quite a few parts of the synth wherean intermittent pot or switch can change the tuning.
The white multi pin headers, as well as the molex connectors used in roland synths seen to go intermittent (probably due to the fact that they are not gold plated, plating gets corroded and oxidises)
I have also fixed a roland rs505 which would lose voices, by cleaning (isoprophyl and deoxit, and reseat connectors) the voice cards.
Maybe any techs on the site can verify my findings, but i think it is a good start (if you are comfortable doing it) to clean and reseat the ribbon cables on this synth.