Most unusual that diagnostics cannot identify a fault. We would have thought that the garage would have at least identified if it was a fuel or a electrical problem. At this stage would suggest that the CKP (crankshaft position sensor) is checked /changed.

If the ECU is suspect then it may be worth removing and sending to a specialist who will check it out. Much cheaper than replacing and also makes sure all is okay. i.e ecutesting.com