I don't think it's the dive flap. I think it's the effect from the boosted ailerons that you're noticing, and the same stick movements that you can do in a J will sometimes cause the L to stall.
That added roll rate, while really nice when at higher speeds, causes havoc if you're not used to it when maneuvering at lower speeds. The J feels smooth, whereas the L does not.
However, if you take some of the dedicated L fliers, they can move it around just as smoothly as some of the J fliers. But put some of them into a J, and they have similiar problems, albeit not as pronounced as moving from a J to a L.
A chart of the roll rates (something on the Internet - can't verify the veracity):

Another chart, seemingly of a later model J. At 100mph, the boosted ailerons make it 2 seconds faster in roll rate to 90 degrees than without the boost turned on:
