Many things could be happening. One important factor with diesels is that many are significantly less powerful when cold. This affects vacuum significantly as it is created by throttle position (I am presuming a 1980 or later as early 240D trannies don't use vacuum). Vacuum should also be tested to see if it is consistent.
An additive designed to free up valves could be valuable. Its possible that the valve body is in poor shape. Sometimes loosening and retorquing the
VB will stop problems like this that can be caused by internal leakage or warpage.