I've recently bought a 2012 320d ED (E90, not the new one) and twice in the past few months, I've been driving along at 70ish and the reduced power light has come on.
The first time in month one, I was climbing a hill and overtaking - lots of acceleration. After a few moments easier on the pedal, the light went out and the warning disappeared.
The second time, yesterday, I was cruising along in 6th on the motorway. Light came on, beep, with no other symptoms. After a few miles further driving, the light went away again but I later noticed, at slower speed, there was no pull from 1500rpm as there would usually be.
On both occasions, restarting the engine brought everything back to normal.
Any ideas on what it might be, or whether it's time to take it to BMW, or an independent? I'm inside third party warranty for the next three months with the dealer who sold it to me, but I'm not their biggest fan and don't feel too confident leaving it back with them.