94 Nissan Sentra transmission problems, only when car first starts up

When my 94 Sentra first starts, whether the engine is cold or warm, if I put it in drive or reverse & I press on the gas, the engine revs as if still in neutral for a few seconds, and then engages suddenly. If I shift into drive and just let the engine idle with my foot on the brake, it will sometimes engage by itself after a few seconds. It only does this for the first few minutes of driving, and then shifts without issue until I turn the engine off for longer than 10-15 minutes.

Any ideas?

Start with a check of the transmission fluid level.

Already checked. Fluid is fine, though I’m getting a fluid change soon, as I haven’t gotten one in a while.