sprinter 312 wont start but will on tow

[sprinterguy]

Hi lads im new to this site 1st one ive joined, I have a beavertail sprinter 312, problem im having is wont start on key but will on a tow and will run all day driving perfect, battry and starter are perfect , ive also put in new glow plugs and tested their woking, also cracked the injector's and seems to be getting the fuel, how ever it had been starting on and off the past month on the key since i replaced two of the relays under the drivers seat any time i replace them or mess about with them i seem to get it starting ive no idiea what there even for just i heard a strange noise out of them b4 and decided id try change dem and ran for a few wks no problem then 1 day i was out on a job in it and wudnt start back up have tried all above since and now will only start on a tow, any one had simular problems or any idieas would be much apperchiated thanks this is a 1996 p reg sprinter 2.9 diesel