If a speed sensor or lack caused it to cut out, I would assume it would also cause it to cut out all the time.
Has it just started or any other symptoms ?, if al you need is to register a speed input there is a hell of a lot eaier way to do it than change the box!!
Some ecu do use a speed signal but this merely raises the revs when coasting, to around 1300/1500 for smooth gearchanges, but does not elliminate your base idle which should keep it going when coasting.
Does it tick over ok ?
tabetha