Clean the innards of your freewheel. 99.99% of the time that's what causes chain droop when back peddling or coasting. Most of the time taking it off and drooling light oil in between the inner body and outer part that has the chainwheels on it will loosen it up. They are a PITA to take apart and get back together - spring loaded ratchet pawls are the hard part and there may be a jillion tiny loose balls inside.