Before you go upgrading, let’s talk about why you see this so you can potentially learn from it and solve it quickly.
You state that you run 2 sets of blades and then the third has this problem. You’re running those motor at probably maximum available current drive given the pot settings of the Gshield by doing a full depth cut. What this means is that you’re overheating your drivers on the Gshield and it takes three consecutive sets to see the problem. I believe what is happening is that you think it just keeps cutting straight but really what’s happening is you’re missing a whole bunch of commands due to thermal shutdown of the drivers and when they finally cool off enough to run, it keeps going but the position is out of sync.
So, you have 1 real option here to solve it: Reduce the load on the steppers which will reduce the load on the drivers and not drive them into thermal overload.
You do that by lowering your depth of cut as Phil has suggested but by lowering your depth of cut, you can also increase your speed so you can usually gain a cut time advantage by doing that.
Try running at 60 ipm and a 0.075" DOC. At your original 0.30" cut depth, that is only 4 passes but at 5 times the speed. You should gain an advantage in cut time and reduce the thermal load of the system. I’d say go 0.08 but that is still 4 passes and a the 0.005" just alleviates more strain (not much but a little).
Try this before going off and upgrading your machine. If you still feel the need to upgrade, then do it. But do it slower and keep your machine running and spitting out fan blades.
Also, make sure your Gshield fan is clean and working!