I don't think this is governor related.
But your rev-limiter might be acting up. There have been a few people with W124s who have had that problem lately.
Does it only do this in 3rd gear? Does it only rev to 5k in 1st and second gear also? And is it exactly at 5k or whatever? Or is the "bogging" down gradual? (IE, starts at 5k, and goes to 5500 when it's really bogging). And when it hits that 5k mark, is it a pattern "bogging" down? DOes it let it rev and then cut, rev then cut? I am curious because this may help determine if it is your reve limiter.

But I don't think it sounds like a serious powertrain issue or anything like that.

Good luck and welcome!!

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)