I factory-test these on an old Dell 755: to give a hint at the age, service tag says support ended 11 years ago. 14 sticks per machine, 400MHz.
Don't know if you have multiple USB busses on that machine, but if you do, split the load up. I run seven per bus.
Thanks for the reply! Were you testing on windows or linux?
Windoze Sux. Use Linux. Had similar issues with my R606's on Win7. Switched to Ubuntu and issues went away. Then switched to the Pi 4 and all was good. Don't know how 18 sticks would fare vs. my two 606's.