Rev to 1000RPM, then release: Engine will drop slightly below 600RPM, stay alive, without hesitation. Then rises back upto 6000RPM.

Rev to 1500RPM, then release: Engine will drop to 100-200RPM, hesistate, but stay alive. Then rises back upto 6000RPM.

Rev to 2000RPM or more, then release: Engine will hesistate, and die.

After dying, only with the gas pressed will it start again...

Anyone seen this problem, or experienced it before?
Like I mentioned, it starts and drives fine.

Thanks.