Engine Intermittently will not shift

[Tilted]

2000 RX 300 - original owner, 143,000 miles, well maintained. Transmission replaced 125,000 miles when it just quit one day with no prior symptoms. Now (frequently) the check engine light goes on and off. Two incidents of  loss of power while driving - had to pull over, then after a couple of minutes with car still in drive it was fine. Intermittent problem with failing to shift to higher gears. Mechanic has run the codes but can't seem to figure out what's up with it. Says to bring it in when it's doing it, but of course it rights itself before I can get it there.Reset codes and passed smog. Help!