Yes, i hear the same about the 5 speed box being stronger. Its the same unit used on the 155 V6, 20vt, early gtv 24v, with a different clutch cover casting.
The 6 speed bearing design is a little weaker, and meant to be more difficult to set up when put together (shaft end float etc), so premature bearing wear (input shaft) is very likely.
Yes, 156 6 speed box is the same, and identical cable controls. My GTV dies the same when the box is cold, and there is a cross the gates adjuster on the gearbox pivot that can make a difference to this. The GTV always goes in if the shift is pressured towards the driver, which suggests the cross the gate movement is not quite pulling the selector fork far enough over. The adjuster on the pivot can be moved a mm or so to correct this, although it might be wear elsewhere causing the issue.
On the GT, the shift moves in 6th and no other gear when you accelerate then back off, which apparently means the input shaft bearing is worn, or the roll pin in the gear selection module is worn. Bearing is easy when the box is iut, so itll have to last until clutch time!
