The Talk of Middletown; 1950's Town Now Battles 1990's Woes
LEAD: At first glance, this seems like an aptly named small city. It is in the middle of the state and midway between New York and Boston. It is still dotted with many of the symbols of middle-class, middle-American life from the mid-20th century: homes with big porches, a roadside diner and a Main Street complete with a striped barber's pole, an old movie marquee and an Art Deco-style