My 400E had very similar problem before, at highway speed, if I kicked down for max accerleration, the rpm wouldn't past 5000 rpm, as soon as rpm reached 5000 rpm, the rpm went into a kind of violent oscillation. ABS and cruiser control also didn't work.
I found out one of 4 10A fuses in the black computer box was shot, after checking the schematic, I found that fuse protected throttle control module and ABS module.
A 50 cents new fuse sloved these 3 problems, the computer box locates at the passenger side just behind the firewall.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.