Thanks for getting in touch with us. Just to check, this data set has worked fine with qiime defaults, but now 'freezes / hangs' when silva is used, right? This difference greatly help us focus in on the problem, because we can infer that this difference is due to the change in database.
Silva 99_otus.fasta database is 764MB, while greengenes 97 is 137 MB. This 5x increase of database will also greatly increase RAM usage. While 10 GB is enough for greengenes, it may not be enough for SILVA 99. You could try using SILVA 97, which is smaller, or silva rep_set_16S_only instead of rep_set_all which will only include 16S references.
To confirm that RAM usage is causing this problem, you can use the linux command 'top' to view your available RAM and make sure you are not 'running out'. If you use all available RAM, this script will overflow into swap, slowing down the search to a crawl and creating the kind of freeze that you described.
Let me know if changing the database helps!