Check capacitor C1 for leakage I had several of them fail in different units. They fail from short to leaking / act as a resistor. They are 10Uf non polar and I use a panasonic 100V version to replace them.the panasonic is a little larger but can be installed. The capacitor in question is directly parallel to the Battery leads and located close to the battery connection point on the board. Everytime one failed, the unit exhibits the same type of behavior you describing,
