I believe there are two issues to be investigated.

The transmission's shifts are coordinated with the throttle position.
The older models have a "pressure" rod or cable from the (top rear of
the engine) throttle linkage down to the right side of the transmission.
At some point after '82, I believe, this was changed to a vacuum system.
Your car is up shifting prematurely because its transmission isn't
getting the correct throttle position message.

The transmission's shifts are modulated by a vacuum powered modulator
whose vacuum supply is controlled by a throttle linked vacuum valve on
the injection pump. The principle is: the greater the vacuum the softer
the shift, conversely at full throttle there's no vacuum to the
modulator because M-B wants a fast, hard shift.