My shot in the dark:Driving in the hottest part of the day and failure occurred after roughly the same amount of time driving. In failure 2 it died sooner but the outside temp was 34.5deg/c. Hotter than before. Whilst at the mechanic it had sat overnight and was dead cool when checked. Reached operating temperature but the engine bay wouldn't have reached a high temperature when only driving around town.Another 2nd hand MAF was provided and fitted. Lasted a bit longer but the outside temp was cooler. Around 29degC. It's spooky how the vehicle behaved almost identically with the replacements though. It failed almost at the same time of day, in the same place (give or take 10km's) after the same amount of driving time. However, on the trip home it lasted about 1/2 hour longer before failing. The outside temperature was a little cooler which leads me to once again think that the issue is under-hood temperature heats the MAF to a point where it fails. Short drives around town do not worry the car at all.
the temperature problem could come from
1. cracked fusible link BMW E34 Website
2. Motronic goes bad when hot
Does the car have e-box cooling? On my E32 750 the e-box on right hand side engine room always gets fresh cool air from the AC, behind the glovebox xompartment there is the e-box fan which runs with ignition on and switches to hi speed when a certain temperature is reached.
We had a similar problem here with someone in Canada, slowly dieing DME Speed Governor / Limiter
Also read this one:
Motronic model 0 261 200 059 The car: '88 BMW 535is
The Problem: The car suddenly died and even though the injectors were still pumping there was no spark or a very weak spark.
The Diagnosis: All the sensors (especially the TDC and RPM sensors) were good and the grounds had been checked. Studying the archives revealed that the ground path for the coil (thus energizing the coil) was controlled by a power transistor in the MOTRONIC computer. The solder joint for the transistor would break causing the transistor to shut off or give intermittant function. The Fix: I spent $8.45 at Radio Shack for a soldering kit and took the following pictures. I have experience doing this plus I had already bought another ECU. Have a parachute before you jump...
MOTRONIC 0 261 200 059 FIX