Problem starting

Hi i wonder if anybody may have had a similar problem i seam to be having with my 1993 3.9 efi Auto Range Rover, When i go to start her i have to keep the starter turning for at least ten to 15 seconds and it doesn't always work but when its tarts it runs like a dream, then it switches over to gas automatically and it will run faultlessly, but then if i stop and turn off the engine it might or might not start straight away even when it has been standing for for a couple of minuets or over night, i have taken the relays out and tried new ones but to no avail, can anybody shed any light on my problem please