have you checked the battery with a multimeter to see if its being charged from the alternator? check batt with engine off, then start the engine and it should go up to 14v. sounds more like a dodgy earth cable, mine was loose on the battery and every now and then popped off and used to cut out if i was cornering hard, tightened the connector on the battery, problem went the earth points are gearbox mount, top of the gearbox back of inlet manifold and on the slam panel iirc, just undo them all, wd40 the contact area's and end of wire connectors, wire brush them, and re-bolt them back up tightly. also check the entire length of each earth wire because if has worn to metal anywhere and touches the metal the earthing point may jump temporarily and cause it to cut out. you can buy the connectors for the battery from most motor factors if yours doesnt fit tidy that will also cause it to jump, for a temp measure, you can fold up some tin foil and wrap it around the terminal and tigthen the connector around that