conflicting tasks (usb and serial)

Dear list,

I am running two USB serial converters, (which steer stepper motors) as well
as a midi out.
The two tasks seem to conflict when simultaneous and hence crash, although
work fine independently. I will attempt to alternate the serial and midi
output but this seems like a bit of a farmers solution.

G4, OS X.3.9,
doepfer CTM on midi,
usb serial converters (velleman.be)
2 serials on a usb bus, midi direct from laptop,

Any ideas much appreciated