I too still run an A7V333, but the middle memory slot died a couple years ago. Perhaps you have a dead slot? I currently can run a max of 2GB:
1GB | blank | 1GB
Possibly and sure I've seen bad DIMM slots.
If the following fails then sure it's likely you have a bad DIMM slot.
CPU: | blank | 512MB | blank |
You can 'try' cleaning the DIMM slot: Use Isopropyl alcohol, dampen the RAM stick's contacts, fully insert in the DIMM slot with the moist RAM, remove, wipe-off, re-dampen and repeat then wipe-off and allow 5 minutes to fully dry. Re-install the RAM and cross your fingers.