Vehicle is year 1999 Boxer based motorhome , Engine is 1998cc petrol. Stalling occurs usually but not always when accelerator is released. At initial start up it idles fine and does not stall. l have changed Stepper Motor and cleared the only fault code which was due to speedo electronics. Is MAP sensor next to try, is this a DIY task ? Any other suggestions ?

Your symptoms will not be a MAP sensor, a faulty MAP sensor will cause poor starting & poor running at high revs+full throttle. If it is faulty the ecu loads a preset table of manifold pressure versus throttle opening & uses that instead.
DONT WASTE MONEY BY GUESSING.

Also there is no such thing as a speedo electronics fault code in the engine ecu, it is simply not programmed with such a possibilty, l am assuming that whoever read the code misinterpretted it.
If the fault code related to engine speed then it may be related to ur problem & means u need to carefully check the connection to the engine speed sensor, the cleanliness of the sensor & that it is mounted the correct distance from the flywheel as it is an inductive sensor. The sensor will not be faulty or the engine would not start.
If the fault related to vehicle speed information then that may be causing ur problem & is much more likely.
The vehicle speed sensor could be intermittently faulty or another big possibility is the 3 wires from the sensor. These wires can often become corroded within a few inches of the connector. The connector sometimes becomes corroded on the power pin which leads to high temperatures & partially burned/melted wires & connector/sensor. l have seen this cause strange running problems before.
However, those motorhomes were fitted with either Citroen/Peugeot/Fiat/Mercedes engines, & the descision about which engine to fit was done on the basis of whichever engine was handiest on the production line! So without knowing exactly which engine it is, it is hard to tell what is at fault for sure. (the Merc engines were not fitted to many)