Spark at the glow plug is not a reliable test.
Get an anolog ohm meter (one with a needle), then test the resistance at each plug through the harness. 0.6 ohms is right.

If that's Ok I'd watch the signal to the glow plug relay from the ignition switch. It may be low voltage or intermittent. In my experience, an original '85 ignition switch needs to be replaced, or you will soon. Some very wierd things can happen when that sucker starts to go bad.

The Run/Start wire is Red/Black. When it goes to 12v the glow cycle starts on and timing. The Violet wire is the cranking signal. It terminates the glow cycle.

So check the inputs and outputs of the relay to make sure that all looks OK.

An anolog meter will show a glitching signal, while a digital meter may not.

Check the battery voltage at the battery with the glow cycle on. It should be 11 volts or better. Should be near that (10.5 or better) at the glow plug relay 80 amp fuse, too.

Checking and cleaning up the ground straps is also a good idea.

A slight hum is OK, but a buzz means a defective relay or a bad signal (low voltage or intermittent) or power to the relay.

Best Wishes,
Bruce McCreary
(2)85 300Ds 83 300CD