So on mission 3 when you have to drive to the Valkyrie it just keeps driving away and I can't catch up. soooooooooo wtf?
Did that to me, I had 1 enemy left hidden, take a look round for 1 use the missile launcher weapon and find it then destroy it then the ship will stop.
I am having the same problem! Restarted three times now and same thing. I will do what you said and search around for a hidden enemy.
If not, mine froze on mission 4 or 5 it is the tank would not move for me when I advanced and have had to start a new campaign.