how can i fix it? transit 2005 90hp lwb?

my transit has started using alot more fuel than normal..firstly when i was driving it at a steady constant speed in top gear, it cuts out on power for a second and the glow plug light flashes on and off, a turn of the key off then back on stops the flashing but not the fuel consumption. i put it in for service which included fuel, air, and oil filter including oil change, problem still there. plugged into diognostics, came up with main relay malfunction? any information would greatly be appreciated. jonny