I previously had a '86 Audi Turbo that exhibited a similar issue, which was quite frustrating. Once initiated, it inexplicably continued to operate. The issue was identified as fractured solder connections in the FPR, similar to the one mentioned above. I deduced that the resistance at the contact site progressively deteriorated owing to pitting, resulting in overheating until the solder oxidized, became brittle, and detached from the PCB. I believed that after the engine was operational, vibration facilitated the temporary connections of the pins. Re-soldering and cleaning the contact points resolved the issue thereafter.