I recall reading that the problem with the transmission was that the early engines were underpowered, 585hp for mk1 and 650hp for mk 2 or 3 so the drivers have to rev the arse off them to get the big tank moving quickly. Once they got the 720hp engines they didn't have rev the bejesus out of them so much and transmission problems eased up. On a related note the Vickers MBT had the same engine and transmission combo and Kuwaiti Vickers MBTs drove from Kuwait to Syria or Jordan under their own power on their own tracks to help with the Yom Kippur war, without engine and transmission problems.