I've had similar problems before. I can think of a couple of things you may check.
1. Broken tooth on flywheel gear. You only hit it once in awhile.
2. Weak starter solenoid. Does not fully engage starter motor with flywheel.
Next time you get the grinding pull the starter before you start the engine. If a gear tooth is chiped or broken it should be visable.
For the second case, pull the starter and see if the edge of the starter gear is worn where it first engages with the flywheel. This could indicate your starter is not fully engaging with the flywheel.
2000 Jeep Grand Cherokee V8 Stock
93 YJ I6 hard top (really belongs to SPEED)
RE 4.5" Extreme Duty Lift, Advanced Adapters SYE, Tom Woods CV Drive Shaft, EA589 10" rims with 33" x 12.5" MT/Rs
Bullet Proof Front and Rear Bumpers and Tow bar