Really does sound like you just have too much coffee in the PF.
Try a little less coffee - small mound, loosely filled...tap down twice..scrape off excess with something like back of a knife or short plastic ruler, then tamp down firmly.
Take note of the position of the tamper in the basket once youve done this, so that you can adjust up or down according to how it fits in the group head, and of course, how it pours.
Insipid tasting coffee could be purely the result of the grind being too coarse and nothing to do with the amount in the basket.
Also take a close look at the puck - if theres an imprint from the screen, and the puck is firm and relatively dry, with no signs of water channeling, its probably ideal.