Those bubbles in the pump basket are staying there because the flow isn't great enough, which confirms what you said about low flow. Are we sure the pressure gauge is good? If the pressure is actually high that would be leading us down the wrong troubleshooting path. That's a long shot. On the filter valve, if you have a 'closed' position, try it and fire up the pump momentarily and see what the peak pressure reads. What make/model and HP of the pump? Also what kind of a filter....I just assumed sand. Doesn't matter though. Eventually you'll resolve this. The fact that you don't have bubbles out the returns now is pointing to a suction side blockage but that is a mystery too because you had a skimmer basket in place.
Al