My Mercedes Benz CLK 500 2004 slowed down and wouldn't accelerate for a few minutes then picked right back up. What could this be?

Sounds like an intermittent drop into limp-home (reduced power) mode. This can be caused by an interminable number of things, some bad, some not so much. If it happens again, you might want to look into it, but I would let it ride, so to speak, until and unless it happens again or unless the MIL (malfunction indicator light) or CEL (same thing, check engine light) lights up AND the light is flashing. A lit light isn't so bad, but a flashing light means get it fixed soon/now or you're risking engine or most likely catalytic converter damage. I had a limp-home intermittent in my V8 Audi for several years…drove me nuts but it wouldn't light the light….finally put a laptop on the OBDII system (with a dealer tech) when it was limp and the diagnosis was an intermittent failing knock sensor. There are two of them on Audi V8s and one was off and on bad…when the car was shut off and restarted the limp mode would go away as the system reset..yet when the knock sensor (probably went too sensitive after 10 years) triggered, it went into limp mode. Replacing the knock sensors was about $600 including parts because the intake manifold had to be removed and replaced. Your Mileage May Vary, of course!

Hi Thank you for your prompt resonse. This is the first time this happened to me. My mileage is almost at 80K. None of the lights came on but I will see how it rides going forward. Thank you so much for your insight as this was very informative. You seem to really know your stuff. :)

Crankshaft position sensor possibly. My Altima's went out. Thats how it acted. Kinda hard to start too. My clk 500 did the same thing coming back from dallas on the highway. Only once tho. Monitoring it now as no diagnostic code or malfunction came up on the dash either. Seems to be a popular replacement part for these cars.