Here's something that might help some of you with this. I found that I, too, was having a problem with keeping the custom setting for the Internet Mail Provider (IMP) using the slider alone. It would change back to High just by clicking from IMP to any other Provider and back to IMP.
I had to click on the Customizer button. Checking on some of the items under the various tabs would not make the slider stay on Custom when clicking on other Providers. It always returned to High.
To get the slider to stay on Custom, I had to click on customize button on the Heuristics tab. When the new box opened, I left the default settings (only one item not checked) as they were and clicked OK. Now, the Custom setting stays no matter what else might be done.
I hope this might help some of you who may not be doing enough to keep the slider from returning to High after you have set it on Custom.
By the way, I am using W98SE, avast4 Home (of course), ZoneAlarm, various anti-malware programs, and a couple of cleaner programs for ridding my computer of junk files saved by IE.