Same thing happened to me with kids in the van, on the highway in
Montana, 60mph and -5F.  Got home with a paper towel roll duct taped
to the control lever pushing it away from the dash.  when I got home 8
hours later, my mechanic said that the problem was in the ignition,
strangely enough, and when he fixed it, I'd say he was right!
I can't remember exactly what part it was in the ignition, but that's
where it is.
-macjack