I had a similar issue that was caused by the manifold (boost) pressure sending unit vibrating loose. I got curious and briefly took it out. Upon further investigation, I found the aluminum threads were all but gone from the steel in the sending unit. Eventually I helicoiled it. Both out and loose caused the slightly low, slow 'fluctuating idle' that you describe. From that I surmise other manifold leaks might cause the same thing.

ALSO, there is a factory ALR test procedure I'm looking for in the FSM. I'm pretty sure it's too big to post so when I find it I'll send it to you.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!