'86' Mercedes 260 SL Question

I have a neighbor who owns an 86 mercedes 260 SL. He's had it into the shop several times, due to an intermittent problem. Sporatically, he experiences
a loss of power, and the vehicle will not go above 30mph. Just as mysteriously as it happens, a few miles later, it clears up and drives as if nothing is wrong.
When he started the engine for me, I noticed that the RPMs rose about 300, then dropped back to idle again. The idle change wasn't like a rise and fall from bad carburetion, but was more like the rise an engine, compensating for an air conditioner switching on, would be.
Does anyone know the problem, or has anyone had this problem that could give me some advice?
Thanks,
K Berg
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There's a relay that often causes such otherwise inexplicable symptoms, it's called the "over voltage protection relay"; it would be my first suspect.
The car is of the right age for its failure.
I believe this car is a 560SL (V-8), not a 260SL.
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