What would cause an intermittent blower fan?
 

1991 300d, 260k

The blower fan was intermittent on my car. Sometimes it would work, sometimes it wouldn't. I think the control panel inside the car is fine (maybe not, but I don't think that's the problem).

Thinking it could be the brushes on my blower fan, and with no brushes around, I bought a new blower fan and put it in. I tested it before reassembling the car, it worked like a champ, so I reassembled everything.

I took the car for a drive and.....no fan.

Could a regulator be intermittent?

Yes, but possibly loose wires or bad ground somewhere. Replace the fuse while you're at it. Does the blower run off the fuse in the fuse box or is there a separate fuse in a little box attached to the driver side strut mount?

An easy point to check the control panel is the wiring by brake booster where it connects near the wiper assembly. IIRC there's a yellow wire with a 0-7V control signal to the regulator.

Sixto
05 E320 wagon 219k miles
99 SL500 69k miles

I had previously checked the fuses
 

I previously checked the fuses and they were good; it's in the little box by the brake booster on the driver's side. I replaced the fuse just to double check and the problem still persists.

One thing I didn't do after I changed out the blower fan and it didn't work, that was on the low setting. I didn't try it on high and I should have. Yesterday when I drove the car with the ac on low fan like I normally have it, the ac worked. But next time it doesn't work, I'll also try the fan on high to see if that works.

Might be time to pull the switch panel and reflow the solder joints for the push buttons. They get lots of abuse. Especially if you get it to work on Auto or Hi.
Further than that Arthur Dalton has great write ups on the various resistors/capacitors that fail on these CC units. I had to replace a couple of capacitors in mine.

124 uses a pwm module to control fan speed.
The porcupine is under the wiper motor assembly.
If it is packed with grime, it will be intermittent.

Also, new fuses wont help if the fuse holder is oxidised.
Get some silver polish and clean the fuse leaves.
What happens is the oxidised contacts will warp under load, and disconnect from the fuse.