Lazy shifts on 1991 M30

[91M30]

My wife's 1991 M30 is slow to shift into high and to lock up into overdrive; i.e., if throttle is held steady to hold RPM at 3000, it will not shift unless you back off very slightly on the throttle. Is this normal? If not, what could be the cause.

[PalmerWMD]

Your valve bosdy could be dirty (or worse).

Cheapest attempt at a fix is to get a full transmission flush (not just a pan drop some shops have a machine for this) PLUS a pan drop afterwards.

Excessive problems may need two flushes, depedning on how dirty the pan looks after your first flush

[Professor_Mike]

The delay you speak of is a characteristic of the M. A transmission flush is a good idea if you have not done one already.