Not to mislead you, but maybe it is not the transmission...maybe there is something going on with your valve timing on your X5...I noticed the same thing on my X5 back in November 2008 with 178K miles on the X5...Sometimes the X5 would be "peppy" on acceleration and then sometimes it would be sluggish, but no warning lights. I had my mechanic do a diagnostic on the engine's computer to see if there were any fault codes even though my SES light was not on and lo and behold my exhaust camshaft sensor were intermittently not work and thus the exhaust valve timing would be off and thus make the engine run sluggish intermittently...Anyhow, just make sure you check everything out...

If I remember correctly, the sensor was about $100 (US) and the labor was not bad because it was easily accessible. My mechanic has seen this happened on other BMWs (e.g. E39 5-series sedan with same 3.0 liter gas/petrol engines)

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