I couldn't tell from your explaination, does this happen at steady
driving, or during acceleration to highway speeds?  Does it occur at
lower speeds?  Does weather conditions seem to affect it at all?  Does
road surface affect it (i.e. bumpy vs. smooth)?  What's the engine temp.
when this occurs (i.e. cold-- not warmed up yet or warm--been driving
for a while before getting on highway)?

Tonya
87 Vanagon GL Wolfs
74 S. Beetle