There is a general problem with many plug-ins developed in SynthEdit 1.0x: Many multi-core systems will become glitchy, unstable, or even crash if more than one instance of the same SynthEdit plug-in is used in a project.
In many hosts, there is a simple work-around for this problem: Simply make copies of the VST and rename them, so you end up with, for example,
and so on. Then use only one of each.
This may or may not work in your host sequencer, depending on whether it allows for multiple plug-in DLLs sharing the same VST ID.