I have two 7890 GC configured the same. Both use the same gas supplies. The output on GC #2 is 28 and is as stable as can be. So I know it is not the gases. I have replaced the detector board and two pressure controller valves. I cannot find any leaks. I can open the oven door on GC #1 and the out put will swing wildly. I can open the oven door on GC #2 and the output is extremely stable. Any suggestions? Thanks.