Thanks so much for the test instructions, feel like I am nearly there with it.
It does not freeze in case #1 or #2. It is only freezing when connected to an amp or to a DI box or audio interface. However in case #2 there is some gnarly noise coming out of the headphones lots of buzz and static... Now I just need to find out what would be causing a ground loop, where that would be on the synth to stop it freezing and get rid of all that noise.. Any suggestion would be enlightening.