Glitches and crackle with Hi res Files

Hi, I am using Audirvana (latest version) on a Mac Mini 1,4GHz i5 with 4Gb Ram (OSX 10.11.2). Musi Files on an external USB2 Hard disk.
I have set the Upsampling to DAC max resolution (24/192) and the interface is XMOS USB Audio 2.0; but very often I experiment glitches and crackles.
Is my Mac too low specs or what are the setting that can avoid the problem?
Thanks for your help

Try setting the maximum memory allocated for tracks pre-load no more than 512/1024

Disable upsampling. Consider upgrading the RAM if you want to upsample.

Thanks everybody. Unfortunately I can’t upgrade the RAM in my MiniMac model and I’d like to use the upsampling, because it sounds great with my DAC (Line Magnetic with valve analog section). I compared the Audirvana algorithm to Volumio and Audirvana sounds a bit better , especially with DSD files.
I tried to reduce the memory allocated to pre-load and it seems to work. I keep my fingers crossed even if I am prepared to get a more powerful computer.