After a recent assessment of the nation's social unrest, American Motors officials decided that it was no time to be selling a car called Rebel, as it had been doing. The company's marketing men conducted many expensive consumer-research polls and found a new name for A.M.C.'s intermediate model: Matador, which the studies found meant virility and excitement to consumers. Last week A.M.C. introduced its Matador in Puerto Ricoand ran right into language trouble. Matador, it turns out, is the Spanish word for killer, hardly a good selling point. In an editorial, the daily San Juan Star tsk-tsked: "We suggest that the name is an unfortunate choice" for Puerto Rico, which has "an unusually high traffic fatality rate."