My next step would be cleaning the source. The peak center results at the
base of your traces shows decreasing high voltage step value over the course
of several days. This value should remain relatively constant. Before
venting it, though, you can check the resistance between each feed through.
Turn the source off. Disconnect the feed throughs. Make sure you can
reconnect them as they were. They should all be mega ohm resistance except
the filament and the light bulbs (if you have them installed). If you show
even kilo ohm resistance, you have buildup within the source. If you haven't
cleaned it in seven years, clean it. CO2 in the source makes C which can
build up. Your helium is also not absolutely pure and over the course of
seven years, stuff enters your source. I would appreciate knowing the fix
when you find it.