possible problem !?!

[lord4play]

Not sure if this is a problem or not. Any one know why sometimes my 03 x5 3.0i pulls like a raped ape but most of the time it pulls like normal? Noticed a few times in the past month. On the rare occasion it pulls hard it's kinda like the difference between my 96 stock chevy s10 and my 97 s10 with heads, cam, headers, built trans, and rear end. It is a major difference in acceleration.

Should I be concerned? Or is the trans possible slipping the majority of the time and I just can't tell?
It still normally has no problem getting up to speed it's just it's such a dramatic difference getting planted into the seat when not expecting it.

Also noticed twice when shifting gears it seemed to hit a perfect gear and lunged forward while accelerating.

Any info would be appreciated.

thanks guys