I'm in the process of troubleshooting the usual "Loose connection, security, alternator failure, ASR Failure," etc stalls.
I've taken the ECU apart, checked the pins closely and found no broken solder, (not sure about the ball grid under the microprocessor).

cleaned all connectors (D4 and fuse box plugs) and rebuilt the fuse box

checking the grounds today, I noticed something I think might be odd, let me know if I'm off track.

Started the car and did a voltage check across the Negative terminal to most of the earth wires that I could get to. None of them showed much, if any, voltage, indicating good ground. Checked across the negative terminal to the little ground wire on the ecu and was seeing voltage there 0.25 or so volts) and between the negative and the ECU chassis itself.

My question is shouldn't those points, (ECU chassis and ECU ground) read 0, or at least miniscule, voltage when the engine is running , otherwise it indicates a bad ecu ground, right?
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Just an update. Removed ecu from the ecu tray, ran a new earth wire to the lock atop the negative battery terminal. Idled for 30 minutes and the drove without any issue. The stall and loose connection issue were getting to be a constant issue, with every drive. Now, for the moment at least, gone. Going to fab up a month for the ecu on the side of the battery box now.

Only remaining issue is transient voltage on the engine side (run multimeter between negative battery terminal and damn near any metallic engine part and there is some voltage .15 to .25 volts). Where exactly is the engine ground supposed to be? I'm going to trace down all of the earth cables, but just curious where it's supposed to be.

Last update for now. Day 2, got ecu secured to battery box and cleaned IAC. Noticed that port where crankcase ventilation hose goes into the air intake hose next to the IAC has been broken and somewhat repaired. May get a catch can for the crankcase vent circuit and plug the old broken connection. But, after 2 days of driving, motorway and stop and go, no stalls, no alarms etc. Fingers crossed.

Please update us as often as you can, I'm in the same problem so I'm watching this topic very closely And very soon I will be fiddling with these things too!
Good luck!

I was having the occasional stall when i got the car last month with no warning lights or beeps, just the ASR failure notice. Started right up. So the ECU could have an issue. After getting the car, I went into the battery tray and cleaned ththe D4 and earth's at that end. I immediately started getting the lost connection errors. Must have disturbed some problem in the engine earth wire. By removing the old earth's completely and replacing them, it got rid of all of the stalls. I'll still be watching for ecu type stalls, but have also moved it to the side of the battery tray