Problem with time stretch

When I try to time stretch audio files (aiff, wav) in Cubase 5.5 I end up with disturbance, noise, cracks etc.
The files work initially and it’s after I time stretch them problem occur.
I use and old pc from 2006 with win7 x64, msi p965 mb, 4gb ram, dual core 2 2.2 ghz cpu.
Any suggestions?