I noticed that at the top of my ramp the minecarts seem to slow.... Prior to that point they are ascending in a clockwise spiral (Always turning right). At the top they turn left..... Could be a red herring though. Take a look yourself and see what you think
Your carts are derailing. Watch it with period (one tic while paused). You'll go up five (occassionally 6) flights 1 box/tic, and then you'll hit an upwards turn that you sit on for 8-10 tics. It will roll back to the impulse ramp, smack the wall and roll back to the upwards curving ramp in about 9 tics afterwards. It will then proceed again at 1 square/tic until it does it again.
This is fine for a single cart track where you won't have traffic jams. The version above that modifies the layout slightly listed above does not derail however, ever. It is also slightly faster. It will take 3-4 tics on the impulse ramp, and 1 tic for the other 3. So each half loop takes roughly 7 tics, for 14 tics/4zs. Yours will take 8 tics/4zs, then will spend 18-20 tics on the next level up (5z), where the modified method will only spend another 7 and be 6 zs up.
If you want to prove this to yourself in another way, create a safety shunt 1z up from each of the impulse ramps leading into the turning ramp. You'll find your carts are stuck in the shunts because eventually you derail, roll back, and go up into the shunt. It's not 100% guaranteed to happen every time but eventually it will due to inconsistent physics of track start location when crossing ramps.
EDIT: I should say, however, that you're correct in one item. You're not ending up in a dangerous rocking horse situation like my carts were eventually. I wonder if that has something to do with wooden vs. metal minecarts, as yours have to be magma proof, but I can't see how.