My 2005 Punto 1

Hi, my 2005 punto 1.2 8valve cut out while driving. AA checked fuel & spark & towed me to garage. They found low compression even though car ran perfectly with 50 mpg. They reground a couple of valves rebuilt it & it started even though they didn't really know what had caused it to stop. 2 days & 30 miles later same thing. It stopped outside a different garage, they checked timing & replaced crank sensor but still won't start - any ideas?