1993 Caravan Transmission Problem - Doesn't Move

Please help with insight if possible, has anyone else dealt with this and how did you fix it?

My customer was having shifting problems a year ago so we did a fluid and filter change and they were very happy with the results.

But now a year later they said it made that clunking noise going into gear and then a week later it wouldn't move at all. But just yesterday they fired it up and moved okay all day.

Today went out and let it warm up and no movement at all again.

Could another fluid and filter change maybe with lucas cure this? Can the filter get clogged up that fast?

Thanks

Dale in Twin Falls Idaho