ultrabeat error in signal

I am having a problem with hi freq noise appearing in the signal on ultrabeat during the release phase of env4(amp)

If i have a simple one osc kick drum I get what sounds like a downsampling effect when the sound reaches the release of the env...

There is absolutly no modulation going on... just the amplitute. This only happens if my envelope is under about 600ms

has anyone else seen this before... Ultrabeat is such a great synth for kick drums... its a shame it cannot keep the signal clean.