Celica gen6 gearbox and engine light

[Rbishop05]

I have just bought a gen6 2ltr Gt jap import automatic box it has been fitted with a japspeed exhaust the problem is on the automatic overdrive it randomly starts revving when off but now when  decelerating the engine light flashes and the overdrive light flashes put the foot down again 80% it goes out if not put foot down harder miss fires drops a gear then no problem in till I take the foot of the pedal again happens, randomly not all the time, any ideas 

many thanks