BMW 318d Sport Loss of Power. Any ideas?

[Skippyn89]

After yellow drivetrain warning came on, I’m suffering from a loss of power in 3rd, 4th, 5th & 6th gears. 1st and second seem okay. It’s as if the engine automatically reduces revs when when accelerating, even after the warning has cleared (it cleared itself). Any ideas what might be up? Car is a 2013 318d Sport.