If undershoot dominates (Errors in read-heavy patterns, Burst errors (multiple consecutive bits), Errors increase with lowering VDDQ):
Fix:
Strengthen Processor Termination (CPU Receiver):
Decrease RTT_NOM_RD resistance value.
Ensure Proc ODT PD is strong (low resistance).
Ensure Proc ODT PU is strong (low resistance) for overall termination improvement.
Strengthen DRAM Transmitter Driver:
Increase Dram DQ PD strength (making it pull harder to GND).
Consider Increasing VDDQ: Provides more voltage margin to tolerate slight dips.

If overshoot dominates (Errors in write-heavy patterns, Random single-bit errors across addresses, Errors increase with raising VDDQ):
Fix:
Weaken Processor Transmitter Driver:
Decrease Proc DQ PU strength (increase resistance/weaker setting). This is the most direct fix.
Strengthen DRAM Termination (Receiver):
Decrease RTT_WR (or equivalent DRAM termination settings like RZQ/7, RZQ/5) resistance value. This is the most critical termination fix.
Strengthen Processor Termination (CPU Receiver - for reflected signals):
Decrease RTT_NOM_WR resistance value.
Strengthen Processor Pull-Down Driver (Minor/Indirect):
Increase Proc DQ PD strength.
Consider Decreasing VDDQ: Reduces voltage margin, lessening the potential for overshoot.