Gilligan

How are you boresighting the mavs? on the ground or in the air? I have a much better correlation if I do it in the air at max range.

I would try setting the boresight off the destroyed mortar site right at 8 miles or so. 
 
Once you establish the point track on the truck with the TGP, try zooming out a bit so it more closely matches with the view you get with the Mav seeker. Start your attack run at about 15 miles out so you have time to line everything up.

Single Flare, No intervals just spamming the counter measure switch down button.

Once you've created the markpoint, set it as the current steerpoint and TMS down to "unlock" or zero out the cursor, then re-set your lock with the area track.
 
You can also use TMS down (once the cursor is unlocked) to set an INS track - similar to an area track but even less likely to lose the lock point when maneuvering.