I had the same issue for a long time and was concern driving it on the free way or long distance. So, this is what I did:
1. Got me a ford tauras fan at pick a part for $25 bucks + entry fee..
2. drill 2 little wholes in my T-stat.
3. did coolant pressure test for leaks, which I've found one the size of needle hole or smaller...
since then issue solved!!! Drove it on a 9 hrs trip one day and it was cooled all the way. I didnt worry about air coming in as this taurus fan sucked huge amount of air just on LO setting..
