Engine shifts down when i floor it sometimes

[peacemf]

well to be honest there is quite a few problems with my x5.
But today when i floored it (not kick down)....was at a junction...
i noticed that the car just went down a gear by itself with a bit of grey smoke out the bum (the kind you sometimes get when you do a cold start, but in this case the engine was nice and warm)!

(at which point i took my foot of the accelerator)...
It seems to be running fine right now....
there is no warning lights etc.....

do you think its a major tranny issue or an engine issue?


Im currently making a list of problems for the repair guys anyway so might as well add another one!