this sounds like the exact issue i am having. i installed the 1200lb hd clutch kit about 10,000 miles ago. it started to do this a few weeks ago and its getting pretty bad. nearly identical symptoms to ones described in 1st post. although i would add that on mine it sometimes goes halfway into gear, doesnt make that final 1/2 inch of travel to lock into gear, and will pop out if i try to accelerate. in some cases i cant get into any gear. i have to fiddle around and release/rengage clutch several times before i find enough of a groove to force the shifter into gear. i feel horrible everytime i do this.
i had the clutch fluid drained and refilled as it was a bit low, thinking that was the cause but it's made no difference.
