Mecer IVR 1200 LBK inverter back up problem.

Guys my problem with this UPS is when grid 220v ac power is off unit  goes into battery back up mode but then after 2seconds the fan drops in speed the output ac voltage goes up to 239v and not stable. Fluctuate between 235 and 239v(not fast) Frequency doing the same (but faster) fluctuate between 49 to 51KHZ.  The load is a 50w bulb and the dc amps on battery cables is already  3.5amps. when add 100w load (soldering bolt)  DC current jumps to +/_ 9amps (too much battery will drain quicker). Tested all mosfets (multimeter for shorts)  , diodes, all resistors, switching transistors  etc. (OK). Replaced electrolytic capacitors but checked only the 5caps on input of 12vDC for short circuit and value (OK). Nothing gets very hot. On secondary of DC to DC transformer i get 178v (battery volts 13.3v DC) on  ac true rms meter. I have 2 of these units both is doing the same. My battery is a 12v deep cycle (lead) 110ah. Battery still fine. When unit is on grid  with inverter switched on output is 230v AC and 50hz. Charging current is .5amps (depends on status of battery). also replaced 3525 IC. could it be one of those SOIC'S? Is it a common fault on these units?. What can i do still?     PLEASE HELP.