Huge Latency and I don't know why !!!

first of all execuse my english it’s not that good

I am using Cubase 8 now and since I was usin 7.5 also I was having the same problem and I don’t have a reason for it sometimes I am playing a midi or instrument track and suddenly I play and after like 10 sec I am hearing the voice coming and sometimes more than 30 sec.

anybody can HELP me ??

I am using cubase 8, UAD Quad Firewire, Mashine Studio, Avid Artist mixer
Mac Pro Early 2008 OSX 10.9.5
Processor 2 x 2.8 GHz Quad-Core Intel Xeon
Memory 32 GB 800 MHz DDR2 FB-DIMM