2002 2.5 Forrester Auto - not shifting up

[drussell]

My 2002 2.5 XS Forrester auto (68,000km, full dealer service) has begun to refuse to change up from 3rd to 4th when cold. This has been gradually getting worse with it now taking 4 to 5 km before it changes. Even attempts to force it to change with speed and acceleration fail.

Any ideas on the cause - should I bail before an expensive auto fix. My mechanic brother suggests that some cars (in particular Commodores) 'learn' behaviours and the fix required is to disconnect the computer (or battery) for a period of time to clear the memory?

PS - this one has been a bit of a lemon. It had a new short engine at 30,000k due to excessive piston slap.