Sequential circuits samplers

The problem is the follow:

When play up to seventh note of the synth, this works good, but when I
play the eighth note this do not responding. An example, to play 7
time the same note in the synth, when you touch the eighth, the synth
keeps in silence.

This problem is very stranger, but I have seen it in other synths like
ensoniq.

someone have had this problem before?
Which is the solution for this ?

The Prophet 2000 is big synth/sampler with analog filters, so I want
to repair it, for continue enjoying with the special and unique sound.

Thanks