wrote:

> any ideas on what's causing this and what components to check?

Check the engine, check the brakes! Hard to say more than that without
seeing it.

Could be fuel surge in the tank side, but that won't kill the engine
_immediately_.

Could be wobbly electrics. Does hitting it cause it to stop ? What
about driving in reverse then braking ?

If you want obscure, then check for a punctured brake servo diaphram.