'88 Corolla auto, delayed 3-4 shift

[plhunkele]

1988 Corolla SR-5, 4-AF carbureted engine, 4 speed Automatic, 189,000 miles.

When engine/trans is cold it it shifts fine up until 4th gear, but will not make the final shift into 4th - it just whines along in 3rd. Car shifts into 4th fine when it has been driven for a mile or so and has warmed up. On warmer days it may shift into 4th first time out, on cold days, it takes longer, etc. In one instance on the freeway it downshifted abruptly from 4-3 even when warmed. Got off the freeway and on next acceleration it shifted up into 4th normally. Fluid is full and clean. I'm suspecting a dirty/partially plugged strainer only because I do not believe it has ever been cleaned, but I wanted to ask the pro's first if this could be a symptom, before I bother to drop the pan...
If not - what is this a symptom of?
Thanks,
Paul H