Thwaite 6 tonne dumper (needs gearbox repair)

6 tonne Thwaites dumper, needs a new gear box/transmission(I think). Runs perfect for about 3 minutes and then comes under alot of pressure. Gears won't change. Leave for a while and it's normal.again. Perkins 4 cylinder engine, running perfect. Starts on the button. Have a short video that can be sent on to anyone of the dumper running and moving. Hydraulics perfect. Suit someone who has a gear box or just wants a good perkins engine