Usually the directional indicators flash faster when resistance is lessened in the circuit (like a burnt out bulb in the rear), but you've already stated that all the other lights work. Do you have a source (factory repair manual) to reference to determine if the bulbs that do work are the correct ones for each socket? Electrical problems can be tough, and yours sounds much different than what I've encountered. Just for grins, you might try attaching a new ground wire between the frame and the battery (even a temporary wire with alligator clips might help). Just a guess.