the same thing happends to me....i flushed my radiator and i took the botton lip off, my car drives normal temp on the streets but when i hit the freeway goes all the way down to 1/4 or so...it has me a little worried!!

 

when I bought my 97 A4 1.8T in august, the temperature sensor never goes above 1/4, unless something is wrong like when the s belt went. I never get to the midway point unless i'm driving uphill through the pass at 55 with the AC on.