Spider Automatic Transmission Problems

The symptoms you've described strongly suggest an issue with the transmission fluid. Implausible shifting and emergency mode activation are common indicators of degraded or contaminated fluid. When was the last time the transmission fluid was inspected or replaced? Also, has the vehicle experienced any recent electrical work or battery issues that might correlate with your low voltage theory?

The fluid hasn't been changed in recent memory. As for electrics, the battery is relatively new and the charging system seems nominal. What's the likely impact of old fluid here?

Given the fluid's age and the symptoms, I strongly suspect it is the primary culprit. Degraded transmission fluid loses its lubricating and hydraulic properties, leading to erratic shifting and eventual emergency mode activation to protect the transmission from damage. The undervoltage might be a contributing factor, but replacing the fluid should be your first step. While a simple drain and fill can help, a complete transmission flush is recommended to remove old fluid and debris thoroughly. Given the potential complexity and sensitivity of automatic transmissions, I recommend consulting a trusted workshop to conduct the transmission flush and inspection.

Thanks for the detailed insight! I took it to my local workshop, and as you suspected, a full transmission flush and fluid replacement (at a cost of 240€) resolved the issue. The old fluid was indeed heavily degraded. Problem solved!