Sometimes this light is on when I start the truck and will go off after driving awhile, and then come back on again, and then other times it is not on at all, is the bulb going bad?
This is on a 1999 F-150 XLT 4X4 5.4L

 

Idk I got the same truck but mines a Lariat I just flick my dash and it comes back on idk what makes it do that

 

Pull the cluster out and make sure the bulbs are seated tight. They twist and lock in the back of the cluster. If they are in good then I would replace the bulb and see if that fixes the problem. If it doesnt then your probably going to need a new cluster assembly.

 

I know a lot of people that have the same problem. It is a solder joint that gets cracks in it. I have done this fix to three different trucks and it fixed all of them. Did mine 20,000 miles ago and yet to go out. the thread is on here .... https://www.f150forum.com/f75/97-03-odometer-fix-93814/ its very easy to do just tear it out and re-solder it.