Bongo Fury

My V6 petrol automatic has suddenly developed an occasional difficulty while starting from a stop sign or traffic signal. The motor sputters and occasionally stalls. This happened when I was pulling out into a fast-moving stream of traffic. In addition, when the van is in reverse gear, the motor sputters and stalls.

My garage has investigated and concluded that there is nothing wrong.

I'd be grateful for your help here!

It seems that if the engine behaves as you describe, then by definition there IS something wrong .... I suppose what they mean is they couldn't FIND anything wrong. Do you know what they checked and eliminated as a cause? if your garage isn't, Bongo's benefit from attention from Bongo experienced garages. Find one if you can.
Do you mean it happens all the time when in reverse, or occasionally when in both forward and reverse? If it developed suddenly, was is just after a fill-up? Could be contaminated fuel. The mechanical experts on the forum will, hopefully, be able to give better pointers as to cause.