Jumpy Ingersoll

I have a ~10 year old Ingersoll with hydriv transmission, when I slide the drive lever forward to engage the wheels, I always do it slowly. But as I inch it forward it seems to always suddenly buck into motion. What I end up doing is inching the lever forward and waiting 2 second to see if it engages, if not, I edge it ever so slightly more forward and wait 2-3 seconds, then a bit further and wait 2-3 seconds... eventually I find a spot where, after waiting 2-3 seconds the wheels get their power and start rolling without bucking suddenly.

This is a REAL problem as I live on a steep incline and bucking often leads to the front end popping a bit - especially since when you are on an incline if you wait 2-3 seconds between each micro-movement of the lever you start rolling backwards.

Any ideas why this is happening or how to fix?

I have another Ingersoll and it doesn't do this at all, its nice and smooth.