Toyota Repair: 89 Celica OD problem, start of a journey, pressure valve

Question
When driving the car will randomly shift out of OD into third and then not shift back into OD. Have replaced thermostat and ECU temp sensor to no avail. Seems to help if I flick it in and out of OD three or four times at the start of a journey. Also just before losing OD I noticed that my engine revs were higher than normal (100kph=2200RPM, but just before losing OD the revs may hit 2600RPM+)

Answer
Hi Ashley, looks like tranny slipping due to electrical or solenoid control pressure valve and when computer detects fault it will Command transmission to go in limp mode (3gear). it resets when you switch ignition but will come back when problem occurs again. have tranny specialist to scan it. best luck