Hard down shift when slowing down - Help!

[Booiebuddy]

Hello - I am new to the forum. I have a 2001 New Beetle black with about 58K miles. About 4000 miles ago my transmission was replaced. I was advised by the transmission guy to drive it for about 3500 miles to break it in. He also said along the way I may have a hick-up or too which was common. Since I have had this back, my car will sometimes down shift hard from 2nd to 1 st gear when I am about to slow down. This happens everyday for me when I am driving home from work, I have a very nasty commute and sit in bumber to bumber traffic 4 to 5 days a week. Within 20 to 30 mins into my commute home, I feel the jolt. Took car back to transmission place, and the car ran perfect. It never did to them. They told me they could not duplicate traffic nor would they know where to begin to look at my car since everything else was perfect (BTW it did the next day at home). I know something is not right, but I cannot make the car do it on command. I feel as though they do not beleive me - Any suggestions???