We have a 7890A GC with MMI on the back inlet with backflush. We are not getting reproducible results. Some injections yield ok analyte areas and them we have an injection with nothing but high background. I have changed the liner, the syringe, the merlin, split vent filter trap, column, connectors at T-union to no avail. There is no air/water leak and the tune is consistent with previous ones. Any ideas? Thanks.