For the records.

1. Check strip fuse first (30 amp outside fusebox) (should not be more then $2)
2. Check your ignition. Wiggle your key just a little.

Yes I know what I'm saying. Weird it may sound but my blower will quit sometime and i checked everything that make sense including strip fuse, bench tested blower motor and regulator was working fine too.
One day I was at my Indy and mentioned him this that my blower just cut itself from nowhere and it does sound like temperature based.
He said let me show you magic and he turned key in ignition just a little and blower started.

Since then i just have to wiggle my keys in ignition whenever blower stops intermittently and it never failed me.

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1) '04 S500 Gold/Beige......120k Miles
2) '93 300E 2.8...Green/Tan MBTex..........202k Miles(and going)
3) '89 420SEL...Light Blue/gray Leather....155k Miles (Retired )