A833 - are they usually a hard box to drive?

As mentioned above, check the gear oil first and make sure it is full and not ATF. Another thing that I had issues with in the past is the pilot bushing, it can be too snug and when it warms up it will hang onto or drag the input shaft when it gets hot. I've also had a pressure plate go bad and when the car warmed up it wouldn't fully release the clutch which made it very hard to shift and get into gear.