Occasional starter grinding

[Texasdoc]

Usually my cobra starter works fine. However, about one out of ten times the starter sounds like it doesn't "catch" the flywheel. I get a grinding sound, then when I let off the button (pushbutton start), it sounds like the starter is free-wheeling for a few seconds as it spins down. I can immediately hit the starter again, after the free-wheeling stops, and it will engage.

I have looked into starter problems online. Lots of references to shimming Chevy starters, and that Ford starters don't need shimming.

Any ideas?

PowerMaster Mastertorque 9604 starter.
157 tooth steel flywheel.
Tremec T5 tranny.
Stock Aluminum Bellhousing.