I think my tension problem is just not quite being used to spinning again. I have a castle wheel with a screw adjustment to tighten the tension. I like to run as tight a tension as I can manage in order to keep from overspinning, and I think I've just been too aggressive for my current skill level.
A: What kind of wheel do you have? When you say a castle wheel is it a double drive? That would have one drive band that goes around the drive wheel twice, once on the flyer whorl and once on the bobbin whorl. A single drive wheel has a drive band that goes around the drive wheel and the flyer whorl, and then a separate band that goes over the back end of the bobbin.
It sounds to me as if you need to slightly adjust everything. If you keep your tension very tight then it becomes harder to treadle, and you must treadle faster to keep up your momentum. If you treadle too fast then you get over twist unless you tighten the tension which causes the treadling to be harder.
If that seems to be the case then you need to reverse the cycle. Try this with an empty bobbin and a leader that doesn't slip. When you put the bobbin on the flyer make sure that it spins freely before you put the drive band on it. Also make sure that the large drive wheel spins freely when the drive band is not on it. If either the drive wheel or the bobbins are sluggish then you need to see what is keeping them from moving freely. See if you can fix it. A good oiling may be all you need, but if the wood has warped a round file will fix the inside of a bobbin.
When everything spins freely then put the drive band on making sure you are on your slowest drive speed. It would be the largest drive whorl on your wheel. Then pull the leader through the orifice and hold on to the leader. Now, reduce the tension on the drive band so that the flyer slips and doesn't turn when you treadle and hold the leader. Starting at that point you want to slowly tighten the tension on the drive band, while treadling, until the flyer turns and doesn't slip anymore. Now you may need to let go of the leader and let it completely untwist. When the leader is untwisted, check the take up while treadling. This should automatically adjust the tension on the bobbin if it a double drive wheel. You should feel a gentle take up when you move the leader toward the flyer.
If it is a single drive wheel with a separate bobbin adjustment then begin adjusting that knob slowly like I mention in the last note. You may have to go back and forth between adjusting the flyer and then the bobbin. Do all of this with just the leader.
Using a lighter take up tension, will require you to learn to feed or move the yarn into the flyer, instead of having the wheel take the yarn from you. At first it will feel strange but it will allow you to have a greater degree of control over your spinning and to change sizes and amounts of twist when you choose.