Entrance lens zero voltage

Hello everyone. 

I have a GC/MS but I am having issues with the HP 5973 MSD. At first, it was showing low abundance of mass 502 and irregular repeller voltage ramp. After two cleanings, I changed the repeller and two types of insulators (repeller and the one closest to the quad). This improved the abundance of all masses, but lately the autotune sets the EntLens to zero, and if I try to do a manual tune, the abundance of every mass drops to 25-30 counts, with a lot of noise (no gaussian curve whatsoever). We checked the resistance of the pairs IF/REP, REP/GND and all of those are > 20 M Ohm (good, according to the manual). Does anyone have a clue of where the problem might be? One strange thing we have noticed is that the resolution of the autotune seems to get worse after the unit has been turned on for a while (more than 24 h). Attached you will see the last autotune performed. 

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