The PC420PRW has a speed controller on the front---it's a sliding lever. You might want to try putting it toward the middle. That has the effect of making it so you can't inadvertently go too fast (causing teeny stitches). It also will help you find the "sweet spot" that is where you are working in time with the machine. You'll still use your foot pedal to speed up and slow down your machine as you FMQ, but the slider speed control sets the upper limit and the benefit is that you can concentrate on getting used to the hand motions without having to worry so much about what your foot is doing. You'll learn to recognize the sound the machine makes when you and it are working well together.
As you do more FMQ, try lowering that top tension some, I'm thinking that with it set at 7 you are likely to have thread breakage unless you're using a fairly thick and strong thread.
To check your tension, look at the back of your sandwich at places where you were fmqing straight lines--if you see the top thread poking through so that the bobbin thread is just laying along the surface of the backside rather than being pulled in, then you need to tighten your top tension. If you are seeing some eye lashing on curved areas but the straight areas look okay then the problem is more likely to be that you were moving your hands too fast for the machine to keep up as you went around the curves--in that case the solution is to slow down your hands, or speed up your machine or slow down your hands and speed up your machine.
If you find that you can't lower the top tension without getting more of the same tension problems, it may be time to invest in a 2nd bobbin case that you can dedicate to FMQ. That way you can loosen the tension on the bobbin case without having to re-tighten it when you want to do regular sewing.
Also, if you get tired of making quilt sandwiches to practice on I'm a big fan of using two pieces of felt sandwiched together. I keep the cheap craft store felt on hand for when I'm working out a new design or just feel like doodling. You don't need to put batting between them, just grab two pieces and go.
Rob