Intermittent Decreased Sensitivity/Loss of Peaks

I have a 7890/5975 system that is tuning fine, baseline is normal. Problem seems to be the GC. Service mode Inlet Leak test shows no problems. The instrument will give me a good run or two, then extremely decreased signal or no signal, or just a few larger molecular weight peaks. The problem is not the samples. I have repeatedly changed out everything in the inlet, changed the column, changed split line, split vent trap, split vent line, septum, ferrules, cut the column in the inlet, cleaned out the inlet + liner, etc. etc. Foreline pressure has been consistent and fine, no vacuum gauge on the diffusion pump but no leaks on the tune reports. Has anyone else had an experience like this? Ideas? Thanks.