Weak or broken springs, damaged cam or lifters, burned or damaged valves, worn out valve guides, to mention a few. Check the compression. If the valves are damaged, the evidence of this is generally pretty consistent. Back when I used to build big block chevy racing motors, we use roller tip aluminum rocker arms which were offset from the center of the valve stem and would impart a turning moment on the valve when opening, giving the valve a small "spin" and keeping the seats (triple angle grind) from getting pounded flat quickly (big heavy springs). Don't know if this goes on inside MBZ motors. In fact, now that I think about it, I have no idea why I mentioned it at all. DUHH!! Anyway all that other stuff I said at the beginning is what you need to think about.
1987 BMW 325