I've had no end of trouble with mine to get it right.
Here is a litany of actions I took to get mine a lot smoother.
Check engine revs... John is right.. I think 600 is the workshop manual revs.
Check driveshaft and UJs for any wear and play, even a good grease up can make a difference.
Changed diff oil (mine was like margarine) with some new EP90 and the workshop manual says Castrol GTX 10/40 engine oil is acceptable.
Check diff mounting, all three bolts should be tight, although there is no torque setting. (I had three very loose bolts)
Check engine mounts, a misalignment will give a very nasty change. (I had two bushes replaced).
Mine is now fine, the key test is a warmed up gearbox, stationary going from D to R. There should be nothing more than a click on selection. No shunt and only minimal noise.
The BW35 is not a very 'subtle' gearbox, at low speeds there will be some selection noise, on throttle, only a slight upchange noise should be noticable. I have heard a BW65 is much smoother. I don't know if a 65 is easily interchangable with a 35.
I normally lock mine in D2 around town, which it's happy to drive in and can easily pull from stationary. I did also use some Wynn's gearbox conditioner to mine which certainly helped smooth out the upchanges.