_There is a problem reported on the Steinberg Cubase support forum regarding  crackles and ASIO spikes with Cubase 6.0.2.

_I experienced this problem after upgrading  under Windows 7 64 Bit with an RME FF400. I reinstalled the RME FF400 drivers (already at the latest driver and firmware release) , re-downloaded the Cubase 6.0.2 update file in case there was a later modification than I had (there wasn't). Without going through all the steps of restoring system images, deleting, updating from 6.0 to 6.0.1 and then to 6.0.2 and also direct from .0 to .2, testing different projects, deleting preference files, unloading and reloading all plugs etc, this is totally solid in that 6.0 is fine, 6.0.1 is fine, 6.0.2 crackles. The Windows performance meters are nowhere near peaking on any of the cores, the VST performance meter is about 1/2 way up, and not a hint of clicks, drop-outs and so on in pre-6.0.2. After the upgrade the VST meter is rhythmically pulsing - I could fix it by increasing ASIO buffer size by a couple of notches (64 to 128 samples), but we're not talking about the odd click creeping in, it's like constant radio interference.

The problem turned-out to be with the vst plugins. Go to your Program Files folder (it will be Program Files (x86) for Cuabase 32 bit in a 64 bit version of Windows. Open the Steinberg folder, open the Cubase 6 folder, open the VST3 folder. There is a file called Cubase plug-in set.vst3. This is updated with each version of C6. 6.0.0 has a file dated 26 Nov 2010 (on my DVD), the 6.0.1 update has one dated 28 Feb 20111 and the 6.0.3 update has one dated 30 Mar 2011. My problem is with the 30 March dated file. Either of the older folders fixes the problem. If you have .0 or .1 installed, copy this file somewhere safe, do the update then replace the newer file.

More details here http://www.steinberg.net/forum/viewtopic.php?f=19&t=8009