Transmission?

It could be a fault with the transmission control system

Does the D4 light blinks while driving?

That indicates the trans is slipping and if fluid level is correct, the trans could be on its way out.

Check the transmission fluid level first. If it is low, top up and retest.

Allow transmission fluids to warm up first and when trans is slipping, try not to stress it and avoid accelerating harder. That would cause the trans to slip more and cause faster wear of the clutches.

When problem occurs, try manually shifting between gears to see if it works correctly in each gear. When clutches fails, they would not fail all at once. Only control or fluid pressure would affect that.

One of the components that would cause intermittent non or erratic shifting would be the throttle position sensor and it might not throw any trouble codes. Get the TPS tested to ensure it is operating correctly.

Check for trouble codes by bridging the SCS connector under dash behind the glove box. Refer to the diagram.

As a final resort if others fails, there are transmisison additives available that might help to prolong the life of the trans or at least reduce the slipping until you can get it fixed. Check out with your parts stored for such additives.