Mitsubishi Chariot engine management light

Hi
I have a problem with my Mitsubishi chariot 1998 model auto.The engine management light stays on all the time.Engine runs well but gear selection is slow.When I accelarate at high speed,it will select one to three but will not select four.When I remove my foot from the accalarator,then it selects four but again when I accelarate,it drops to three.
Some days back, I removed the battery terminal for a few minutes,then tightened it back,engine light come on for a few seconds then went off like normal.Drove around ,no problem and gear selection was perfect.Parked the car and switched the engine off for about 10 minutes then on and light could not go off again.Very confusing.Disconnected the negative terminal again for a few seconds in the evening,then tightened it.This time,I did not put the key on because I wanted to see what was going to happen when I put key on the following morning.All was perfect,drove the car,switched off for a few minutes and then on and the light couldn't go off.
The last service was done at 94000km and AT F changed at 68000km and now the car has done 100011km and needs cam belt change.
Do you thing it is something to do with the belt and service that is dual? The other thing I noted is,the engine sometimes back fires but only when I switch off the engine.
Please help
Godfrey