X-Head Porting....

Not to be too blunt........ but that’s part of the learning curve.
Knowing what approach/tactic will end up with those results.
That happens a lot less with shorter shanks.

You should try the air again, and turn down the regulator on the compressor pretty low(tank pressure high, line pressure low), which will help stabilze the speed fluctuations.

Many years ago I bought a cheap single stage “5hp” compressor($399 from Harbor Freight in 1993).
Maxed out it would go 115psi.
I put a regulator on the outlet and set it at about 50psi.
When I was doing the heavy material removal....... it would run non-stop.
I ported a lot of heads that way.
I still have it in the shop as my back-up compressor

At 50psi, the grinders generally don’t have enough torque to bend the shanks. They just stall out.