You shouldn’t be skipping steps. The current can be adjusted, but it depends mostly on the ratings of your motors and a bit on how much cooling you have on the drivers. Keeping things lubed and light helps, but it has so much torque.
The thing that isn’t super obvious is that high speed in the Z really invites skipped steps. Those steppers are turning the screws and if you look at the steps/mm, they are 4x more than the steps/mm of X and Y. So asking 15mm/s in Z is like asking for 60mm/s in X or Y. Ideally they are treated much differently. In your CAM or firmware, you can have your max X,Y speeds and accelerations be pretty high compared to the speeds of Z. Steppers moving fast are trouble for multiple reasons. The torque per step is lower, the microconroller has an increased CPU load, and the forces are higher. 3D carving can be a real PITA with a slow Z, but I’m guessing you’re not carving 4’x4’. So you should probably try to keep Z under 10mm/s, and even 8mm/s can be a lot better. If it’s spending too much time in Z, try to do fewer passes, deeper and slower in XY, which will reduce the amount of Z needed, and you can also try a smaller clearance plane value, which will make it climb less.
Series is better than parallel w.r.t. skipped steps.
Rarely, someone has a loose wire connection on a motor and that can cause symptoms like this. It’s hard to diagnose though.
IIRC, Ryan’s rule of thumb on the current is to set it to about 80% of your stepper motor’s rating. If it’s over about 900mA, or so, I make sure there’s a fan on the drivers. Drivers have a max rating too, but it’s usually higher than the steppers.