TThreadedQueue not capable of multiple consumers?
Solution 1:
Well, it's hard to be sure without a lot of testing, but it certainly looks like this is a bug, either in TThreadedQueue or in TMonitor. Either way it's in the RTL and not your code. You ought to file this as a QC report and use your example above as the "how to reproduce" code.
Solution 2:
I recommend you to use OmniThreadLibrary http://www.thedelphigeek.com/search/label/OmniThreadLibrary when working with threads, parallelism, etc. Primoz made a very good job, and on the site you'll find a lot of useful documentation.