donald:

my friend's 84 showed almost identical symptoms 2 wks ago on a trip
from michigan to tenessee.  the problem ended up being a finely cracked
primary coolant tank, which was allowing air into the system, thereby
displacing coolant into the fill tank, and ultimately onto the ground.  at first
the symptoms were identical to yours (usually ok at freeway speed,
overflowing fill tank at gas/rest stops), but after awhile the problem
occurred no matter what speed.  good luck.

matthew