Originally Posted by Exit93
So I don't think its the boot. It still pops out even when it get warm. Wouldn't it pop out of other gears too if it was the boot?
not always. The way everything lines up in the truck, first would touch the boot further than third and fifth but you should also see improvement with the boot warmed up.
Another possibility, have you recently done a body lift or worked on the rig at all?? sometimes that kind of work can shift the drivetrain just enough to touch the trans tunnel hole for the shifter.
If neither of these are the case, it sounds like the internals, as others have said above.