Mozart, Rondo in D Major (K.485)

I have seen this LH pattern in a few pieces, most often in Mozart, and it drives me crazy. I'm speaking of the LH pattern that opens the piece and recurs throughout.

I'm having a hell of a time keeping those repeating quarter notes held for their full values. As I alternate between the eighth notes above them, I tend to release the quarter notes too early, and I basically turn them into eighth notes themselves.

I'm using the fingering 531 for the D, F# and A of the first measure, then 421 for the E, G and A of the second measure, etc. In both cases, the finger holding the quarter note (5 and 4) always wants to come up too early. It ends up sounding too separated and it drives me crazy.

This only happens when I bring the piece up to speed. I have played it very slowly for ages, and when I take it slowly it sounds perfect. As soon as I increase the tempo, however, those quarter notes don't get their full value. I dont know what to do besides continuing to play it slowly, but I almost feel like that's no longer helping.

Any tips?