One suggestion for diagnosis would be to install a small light in the passenger compartment to tell you when the glow plugs are energized. Any small 12 Volt lamp connected between any one glow plug and ground will illuminate when the glow plugs are energized and go out when the relay clicks off. If the light comes back on while driving then you know the relay has clicked back on, even though it should not.

The relay should not be able to re-energize until the ignition switch has been turned off and then back on. The famous "violet wire" from the ignition switch to the preglow relay tells the relay that the engine start cycle is complete. The preglow relay then turns off. If this wire or the contacts in the ignition switch have a problem, the pre-glow relay could be fooled into thinking it's time to glow.

The "glow light" I describe above will also let you measure how long the relay is staying on, if it is staying on. Early 124 preglow relays without afterglow are supposed to turn off after about 30 seconds. If the light shows you that the relay is turning on and timing out after 30 seconds, then turning on for another cycle a random time later, the problem could be in the violet wire circuit. OTOH, if the preglow relay is turning on and staying on, the problem may actually be a failure in the relay itself.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970