Transmission Disengages for a Second Over Rough Surfaces

This sounds like an electronic problem and if I had to guess it’s location, I’d say it is in the solenoid packs on the transmission.

The reason I am making this judgment is simple: On nice, smooth road surfaces, everything is fine. However, when things start to go snap, crackle and pop on rough roads, the solenoids may jiggle around and may momentarily lose contact with the other side of the circuit. The result is that the transmission disengages and re-engages. Further, it is also possible that the connective wiring has loosened up from its terminals and the result can be the same.

Fortunately, this is a relatively easy fix. It should cost you about $175 per unit including labor. This fix is replacing each solenoid and its wiring. Since there are usually two physical units (each duplexed — one unit housing two units), the whole cost should be around $350 to $425 max.

Once you gave this fixed, your BMW will be set to go.

One last note, you mentioned this happened when your transmission warmed up. Honestly, what I believe is happening is that your engine and transmission are heating up the solenoid packs and wiring to the point where the wiring may just be coming a bit loose which is also contributing to the problem. Let me know how it turns out.