try pulling off the shroud around your main brushes and check brushes,look for loose wires,mouse nest,etc.

its not your shunts or coils? seems to me thats what it sounds like.

The grinder uses the exciter power. If You arn't getting full power from the grinder check everything concerning the  exciter, including the wires leading away from it.

I think I had the same problem your having.Take a volt meter and check you DC volts at the plug.You should have about 30 DC volt at Idle then 100 at high idle.What I did to fix mine was with the machine running took 400 grit sand paper to the armature.I replaced the brushes,spings and coils and still nothing.With the machine running(idle) if your not getting 30.Put your volt meter in the plug and connect a battery to the studs where the brush bolt down(make sure you can remove it fast)I had to hold mine on there for over a minute!!!You must use the volt meter so you don't blow up the battery when it starts making higher volts.

This is how I was told to test coils:

Small leads should have 180 ohms between them(set ohm meter to highest seting)note:will not make toner beep!!
Large leads should have 0 ohms(will make toner beep)

What this is doing is flashing the exciter fields.

Let me know if you need any specs...Been there done that LOL