Corolla 1993 LX trans

May I ask for your expert opinion please?

This is a 1993 Corolla 1.776 L with the LE converter and electric-
switched overdrive. Odometer reads 399,800 km. At least since 2000,
the transmission has not been serviced - it may never have been.

The fluid looks clear red, not burned and usually shows as full, Now it
shows slightly over the full mark (none has been added).

Lately, on an intermittent basis, when starting from cold sometimes the
transmission is slow to shift up. When stopped and put into reverse, it
may thump as the drive engages suddenly.

On one or two occasions when driving at suburban speeds around 70 km/h,
the car has momentarily gone out of overdrive possibly into 3rd or 2nd
gear. Sometimes I have seen the dashboard warning light 'overdrive
turned off' illuminated momentarily.

Can you suggest what is causing this, and what checks I need to make to
diagnose the problem? This car is 19 and an old friend.

Thankyou for your help,