I had the exact same problem when my high pressure fuel pump died and started taking injectors with it, the rail wasn't seeing enough fuel pressure so would crank all day long before it eventually would sometimes fire sometimes not. Brake cleaner would always start it.I just meant, that if an injector is failing there would be other symtoms. Or am i wrong?
I think my repair ended up cheaper than the pump and injectors would have cost lol....As above, do a leak off test on all injectors and also drain the fuel filter (plug on bottom) and inspect for metal filings. If no metal then invest in injectors. If metal then £££ pump and injectors, £700 in parts to repair minimum and probably 2 days labour.