Since it only happens coasting, I suspect that the brake is mis-adjusted, and partly engaging when you coast.

Coaster brakes differ in their mechanisms, but all basically consist of a helical driver called a Bendix which moves a clutch one way to engage the sprocket to the hub shell and another to compact or expand a braking mechanism keyed to the arm on the left side, and engage that to the shell. They should be adjusted so there's a small dead zone in between when neither the sprocket nor the brake are engaged.

BTW- I don't think your bike is a lemon, though the LBS mechanic can probably use more practice on coaster brakes.

The sad truth is that most low to middle range bikes aren't built well enough for every day use. The manufacturers make a calculated gamble on components based on the assumption that the owner will use it for weekend recreation only, and never put a ton of miles on it in it's lifetime.

When it's time, have the rear wheel rebuilt by a competent wheel-builder, using decent spokes and it'll last for years. As other parts die, replace them with better stuff until the bike gradually becomes suited to every day commuter use.