After months of trying, I have finally narrowed down this problem, but it is still with me. Make sure the fluid level is correct by using the check valve located behind the front left wheel. If you don't take care not to overfill, froth can form and lead to pressure problems. - use a multimeter to inspect the wiring connecting the gearbox to the TCM for any signs of a break. — when using a typical on-board diagnostic (OBD) device - locate the part that says "clutch voltage data" The hydraulic pressure solenoid should be around 0.53 volts when the engine is idling and should only rise when shifting into reverse or drive, among other functions. - As a precaution, the selector flashes and refuses to engage (a "lockout") if the value exceeds 0.9. £60–£100 if not get a new valve body... It's possible it's just electrical because mine only starts up every so often, with no problems whatsoever.