If the variations in duration are more than a couple ticks, that pretty much has to be a setting in DTX900 or maybe a problem with the port driver if it's connected by USB. I suppose that would also happen if the tempo of the project is varying. If all else fails, you can easily select them all and use the Event Inspector module in the Control Bar to set a fixed duration for all of them at once. I do this with drum parts recorded from keyboards.