Hi, The following is based on the presumption that the 5HP30 control valve assembly is similar to the 5HP18 gearbox. If it is, then I know what your problem is as the same happened to my 5HP18 gearbox while luckily it was at the gearbox reconditioners. There is a long plastic shift valve in the control valve assembly that sticks when it is (a) worn and ( b ) the gearbox is hot. It needs replacing and to do that, you have to drop the front bottom part of the valve body off. Easiest thing is to contact the gearbox reconditioner I used. He is John Rawle, he is based in Redditch, give him a call on 01564829740 in the New Year. Geoff