There seems to be a school of thought that these gearboxes can suffer from pressure fluctuations caused by either wear or gunk (very technical term
), or viscosity change due to fluid age/abuse or spring wear (or a combination of course) around the main pressure regulating valve.
Whatever the cause, the result is pressure spikes which can cause irrattic changes and can lead to the clutch A drum problem.
The fix seems fairly simple:
Change the Pressure Regulating Valve over for a new one, and couple that with a fluid change.
This can be done from under the car with the transmission oil pan removed (which will be done as part of the service anyway).
Many (including myself) have opted for an improved design of pressure valve made by
Transgo.
Ultimately, my transmission had a broken pipe internally, which caused most of its shifting issues and limp mode, however I now find the transmission shifts very smoothly and I have peace of mind with the Transgo valve fitted.
Cost is pretty cheap - Valve £50, Fluid £70-150 (depending if you use genuine fluid or not), Service Kit £30-50 (ditto), professional installation and basic trans service £200 (approx 3 hours labour).
As an aside, my transmission specialist who knows the 5HP24 well, mostly in Rangies, Beemers and Jags, seems to think they suffer from warping of the valve body, which leads to leaks across the pressure plate (the transmission equivalent of a blown head gasket).