Modern Vespa

I have a 2016 150cc GTS with about 80,000 km on it that occasionally dies. Typically this occurs after braking and hitting a speed bump, or going off a five inch curb, but it does die when simply coming to a full stop as well.

Turn the ignition switch off and on and it will start again. Doesn't appear to be starved of fuel, and the battery has been replaced.

Does the 2016 model have a sensor that cuts the electric? Anyone have any suggestions?

Thanks.

Does it have a side stand that acts as a kill switch like my 2019?

Yes, it has the side stand kill switch and I suggested that to the shop, but I've no idea if they looked at it. I'd left it there and when I returned they said they couldn't find anything and sent me on my way without charge.

At 50,000 km the bike would die sitting at red lights. I'd walk it to the side of the road, wait a few minutes and it would start again. This turned out to be the alternator.

I've also had a three way switch go at 20,000 km, and the usual battery issues, but this is a real puzzle.

Certainly sounds electrical. Loose connection somewhere between the ECU and spark plug?

My big Beo started having idling difficulty after being warmed up. After a fruitless quest for fuel problems, it turned out the spark plug wire was loose - not the side that connects to the spark plug, which of course is pretty easy to look at, but the side that connects to the coil, which is buried under the footrest.

Who knew both connections were important??

nope

Good information.

I'll be travelling for two weeks and will be taking the battery out and on a trickle charge soon. I'll check for loose and corroded connections at that time.

That last sentence is totally incorrect. True, a steel wire of the same diameter as a copper wire would have more resistance - but the frame is the equivalent of several thousands of such wires in parallel. Extremely conductive!

A direct connection to the battery negative can make sense in some situations, where ground loops due to faults can cause other equipment to malfunction. No such situation occurs on our scooters.

OP - I'd be looking at the coil/spark plug/cap connections first. It's very common for the spring inside the spark plug cap to become weak, and make poor contact. This often doesn't matter when revving, but come to a stop especially with the rear brake applied and the vibration (rear suspension locked) can cause the bike to stall.