Why does this happen so often on the left coast?

I have a Tom-Tom. Sometimes I work in the Rio Grande Valley. I don't know if the problem is the proximity to the border or what but it drops signal like crazy, doesn't recognize streets in McAllen and leads me astray. At 7 a.m. in the morning there is no one to ask for directions. I always check Mapquest the night before just see if they are in sinc. This last week I have worked in Seguin and El Paso and it is right on without dropping the signal.