hyundai r55-7a excavator start stop issue

Hi i have a strange issue with my excavator . It all started when it failed to start , checked it out and found that the starter relay was not connecting when excited . That was straight forward enough , fitting a new replacement . also the ignition switch was getting a little tired , so a replacement item was also purchased and fitted . All done and seamed good ! but no ! Now If i turn the key to the first click , all appears as normal , dash lights go through the start up sequence . continue to second pont and the motor now turns over , the motor starts instantly for probably 1 second , then continues to crank but no go . if i repeat it will briefly fire up for the 1 second each time and although it will turn over well , it fails to continue running . I have checked the fuse box and fuses , all are good with power in and out . Then machine has always been a great starter and until the starter relay failed not a moments problem . Am i missing something obvious ? I have tested the new ignition ,The ignition switch has power in , 1st position x2 power out , 2nd position(crank) 2x power and trigger wire has power . Also the fuel shut off solenoid on the pump is getting power and you can clearly here it operating Has anyone ever experienced this issue before , or have a suggestion as to what i should be testing or looking for ? Any help would be very much apricated , regards Glenn.