suddenly the transmission stuck to a gear and do not shift at all??

I own a 2006 ML500 with 81K miles. From time to time suddenly the transmission stuck to a gear (I don't know which one, sometimes gear 2, sometimes gear 4 and sometimes gear 1) and do not shift at all. This happened at least 4 times for the past 5 months. The first time, it just stopped the car and put it in park then I tried to put it in drive and revers but it is like neutral, no shift at all.
Then I turned the engine off then I started the engine on and it started to shift normally as if nothing happened.
one workshop told me it is the transmission brain which need to be replaced for over $2K which is the part only.
Any thoughts??