I need some urgent help I've tried everything.

I had a GC 5890 with an MS connected to a HT3 headspace autosampler and it was working fine. It had an EPC module with manual flow controller. We decided to upgrade to a GC 6890 (EPC only) which was working fine with manual injections but as soon as we connected the HT3, the pressure started having issues stabilizing. It was jumping quickly (~+1psi) above the set point (5 psi) and then slowly decreasing below the set point (~-1 psi). We disconnected the autosampler and fixed any possible leaks our GCMS had alone as it was a new system and then connected the HT3 again but the same thing happened. We redid all of the connections at the heated transfer line and isolated individual valves in the HT3 to make sure it wasn't internal but we've had no luck. I've done soapy water, isopropyl water and I purchased a RESTEK leak detector but had no luck finding a leak issue.

I'm starting to think it is not a leak but possibly an issue with the EPC trying to control the system with so much extra volume due to the heated transfer line into the HT3 ans back. Someone also suggested changing the split vent trap filter which we are looking into but if this doesn't work I have run out of options. I'm thinking it might be a software issue but I am not too sure how to fix it. Does anyone have any tips?