Hi John
I had something similar a few years ago with a smart where sometimes the car would die then not restart and the 3 bars would flash in the cluster. Annoyingly intermittant and every man and his dog had looked at the car to no avail.
I used it for about a week and it seemed to do it most over bumps/round sharp bends, die as above, then restart 5 mins later.
I found if I wiggled the connector to the SAM control unit (under passenger dash) it would start.
So I opened up the SAM unit (you have to bend the 3 circuit boards apart!)and looked at the board with a magnifying glass. Turns out all the pins where the connector plugged into had dry soldered joints! resoldered all the pins (tricky if I remember) and it never happened again!
One VERY happy chappie as he was about to scrap it!

Hope this works for you!

All the best

Colin