Pre-Power Stroke Diesel (7.3L IDI & 6.9L) - Ford Truck Enthusiasts Forums Here is a link to the page for your engine. Note the sticky at the top of the page dealing with glowplug problems.
I would think you have a few bad glowplugs which means that the controller shortens the time the plugs are heated. With an indirect injection diesel working glowplugs can be needed even in warm temps.
However post your problem on that page for more info would be my advice along with testing all eight glowplugs.
Good luck.