The main problem with the JD800 is the keybed.
They made a huge design cock-up by using some ridiculous low melt point red glue that holds in the weights inside the indiviadual keys. This, after time, in a hotish climate (even in UK
) melts and drips all over the damping stripe and contact rail. The keys become sticky and its a nightmare to play.
It can be cleaned up though. Roland JD-800 sticky keys service / repair
The sliders can get dusty, causing parameters to go crazy, adjusting themselves, but sometimes a good hoover and rapid 'back & forth' movement can get them back to normal.