This is normal for a hydraulic shift trans. When the engine has low power, manifold vacuum is low / throttle is opened farther. This leads to higher line pressures / later / harder shifts as the trans thinks the engine is under load but isn't.

This is generally ignition related, at higher engine loads voltage required to jump the gap increases. This can lead to not establishing a spark / spark leaking out through a boot or coil. Once engine load is reduced, the spark returns to the plug tip.

Coil or boot failure is pretty common.

Have you checked fuel pressure driving under load?

I'd hang a volt meter / scope on the throttle position sensor output to see if the throttle is opening enough. MB of this era had ASR ( Acceleration Slip Regulation ) tied into the ABS system. Perhaps the ASR is turning on due to a erratic rear wheel speed sensor. On a MB this can be completly shut off for dyno testing.

2 MV sounds low, measure it at the sensor and see if that matches the computer end.

I'll counter the previous suggestion that the cam sensor causing the run problems. Cam sensors are to sync up the injector pulses for a slightly smoother idle. Once the engine gets up in revs, the injectors are open longer making the sync not an issue. This engine has waste spark where 2 plugs are fired from the same coil making any sort of compression / exhaust stroke ignition sync a non issue.

For engines in general, the cam sensor is only looked at during cranking in order to syc things up so a come and go sensor won't be an issue.