And in relation to Gw's suggestion, I don't think the bike coming to a stop should cause the engine to stop abruptly because once the engine revs drop as the bike stops moving the centrifugal clutch should start slipping.
So, do you think it's killing the engine because there's a wire somewhere that's seizing out because you are sitting on it, perhaps a wire shorting under the seat say, or because the bike moving causes a wire to short out ? Does it feel like the engine stops then stops the bike moving, or does the bike stop moving and then than engine stops because of that ? You should be able to detect that by the sound of the engine, whether it stops firing in a single firing, or whether the bike stops moving and as it's happening the engine sounds like it's labouring.
And, after it's come to a stop, does it restart ? After it's stopped, can you push the bike forwards or backwards (try both directions) ?