96 Dodge van

I've been having some issues with the automatic transmission on my 96 Dodge van recently. It has a V8 motor with a 3 speed automatic transmission and AC.
The transmission seems to slip when going into the top gear, The slip seems to start around 28-30 mph and it doesn't fully engage until around 38-40 mph, lifting my foot from the gas seems to help the shift. It has also started to slip a little when going from 1st to second gear and will occasionally 'clunk' into reverse when I shift.
I've checked the tranny fluid and it seems to be at or near the correct level.
Can anyone suggest what the problem might be?
Best regards.

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