My heater, when asked for heat, comes on and off with no apparent rhythm.

Any ideas?

Does your car have manual or automatic climate control?

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Don't Chrome them; polish them

Automatic

What year is your w123?

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"It's what non-car people don't get. They see all cars as just ton-and-a-half, two tons of wires, glass, metal and rubber. That's all they see. People like you or I know, we have an unshakable belief that cars are living entities..... You can develop a relationship with a car, and that’s just what non-car people don't get."
-Jeremy Clarkson

1985 California version

Unplug the monovalve on the firewall and see if you have full heat all the time. If you do, the CCU is probably the issue. If the heat is still intermittent, replace the monovalve cartridge. Don't waste money on aftermarket parts either, there's a high probability that they won't work.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Thank you.