We were having a smilar issue, only its with motorcycles. Not all motorcycles, just crotch rockets. I first noticed when Vick and I were on a walk; one zoomed passed and for the first time since hes been in my life, (about 6-7 months at the time,) he started barking and going nuts! After I got him settled down he was fine, a few minutes later another came by and it was game on again. What worked for us was sitting there for a good bit and everytime a bike flew passed Id just talk to him calmly and pet him. We have yet to have the problem again, now if I could just get him to stop going after squirrels.