Do you drive a big rig? If not, don't bother. The high gain antennae are huge. The only ones I've seen are like 12x12 and designed for mounting on the outside of a building. I think there was a 'trucker' antenna too, but IIRC that was like the Terk 'flying saucer' and would look goofy on a car too.
Are you noticing the dropouts in the city or just randomly? If buildings are blocking the signal and there are no ground repeaters in the area, even a high gain antenna won't help. However if it is happening at random (or maybe you notice a pattern, like only when it rains, when it's hot out, etc), you more than likely have a failing antenna or the cable is chafed/crushed or fraying internally.