I used to get crackles when I used LE1 and two instances of Proteus VX VSTi. Very unstable and not the fault of the hardware. In Cubase, go to Devices–>VST Performance. As you hear crackles you may see the meter hitting red. If so then unfortunately there is not much to be done. You can search “improve VST performance” or something like that and try the tips and tricks. I ended up moving out of LE to the paid version and it works good. You can try downloading a trial version of one of the paid versions, like Cubase 7 AI which I think is currently available for trial. Then load your VSTi and VST effects and see if VST performance improves. A quicker test is to unload the VST instruments from cubase, if possible, and add them back one at a time to see if one of them is causing the crackles. It is easy to do with paid versions which have the VST Instruments menu choice under Devices. Just delete from the menu. With LE4 I’m not sure how to easily unload instruments. Even if you use no instrument tracks, I think the VSTi’s still sit in memory.
Hope you figure it out.
-Leon