You say you have fitted a new lambda, what make is it as these can be a bit of an arse and not see the lambda working correctly.

I usually fit bosch and then reset all adaptions and not have any problems.

Not seen any sticking valve probs on these for a long time now but a lot of cam timing issues where people have fitted belts or done heads and got it wrong or left the crank pulley slightly loose causing wear.

kelly