That's weird - my ATF dipstick does have max and min marks and the fluid is "properly" visible when you check it. Given that you've checked out the more obvious areas of the fuel system and TOD light has started flashing I'd be looking at the ATF levels.

I know that when I changed mine I (accidentally) over-filled it and it kept holding 3rd gear instead of 4th. Once I sussed what had happened and drained a little bit it started behaving properly again.

If I were you I'd be changing the ATF sooner rather than later. Even if that doesn't solve it you know you'll have ticked off another service item...

Hope that's some sort of help,

Alan