hey Clark,
been thinking about this one...do you know about bypass plumbing for pumps that create too much flow?
I dont know if you can do this with your particular set up, but if you continue to run your return pump choked back, you will decrease the life of it.
one way people have fixed that particular problem is to create a bypass valve in their return plumbing. It involves adding a T to the plumbing just above your return pump. After the T, you add a ball valve, (unions would be a good idea too) and run that plumbing back into your sump. You will also need, (if you dont already have one) a ball valve above the T.
By adjusting the two valves, you can divert flow from the return back to the sump, which doesnt create back pressure on the pump.
Again, I dont know if this would work in your system, but I thought I'd pass it on. Here is a really simple Paint drawing I did to show you what I just said...obviously, you should make your bypass plumbing return somewhere away from the return pumps intake....otherwise you'll contribute to the microbubble problem.
I dont know if this will help you out since your micro bubbles are caused by your pump cavitating....but it might be worth trying...
Nick