Hey there! I totally get your frustration - it's like those servo motors have a mind of their own sometimes, right? Let me break this down for you.

First, about that mysterious drift - it's usually not so mysterious when you look at the common culprits:

1. Temperature changes - Components expand/contract, resistance changes
2. Mechanical wear - Bearings, gears, and couplings wear down over time
3. Electrical aging - Capacitors, resistors, and connections degrade
4. Load variations - Your machine might be handling different materials or weights
5. Environmental factors - Dust, humidity, vibration all take their toll

Now for the systematic re-tuning approach that won't mess up production:

1. Document everything first - Record current settings, performance metrics, and issues
2. Schedule during maintenance windows - Never tune during production runs
3. Start with mechanical checks - Tighten couplings, check bearings, clean encoders
4. Use the 'inner loop first' method - Tune current loop → velocity loop → position loop
5. Make small, incremental changes - 10-20% adjustments at most, then test
6. Test with sample runs - Run test patterns before actual production
7. Monitor for a full cycle - Watch the first few production runs closely
8. Keep a tuning log - Document what worked and what didn't

Pro tip: Consider implementing preventive maintenance schedules that include regular parameter checks - catching drift early is way easier than fixing it after it causes quality issues!

Hope this helps you tackle those pesky servo motors with confidence!