no audio at startup, for a random period of time!

I’m new here and pretty new to Cubase. When I start Cubase, and load an existing project, I have to wait sometimes 10 minutes until suddenly the audio output starts working. Sometimes it works right from the beginnIng. The sound card appears in VST connections setup, from startup. And the sound card is working fine with other applications, so this is strange. What could be the problem?

More details:
PC is Dell Inspiron 1501 with Windows7 “ultimate”. Processor is setup to give priority to background processes, as recommended by Focusrite.
Sound card is Focusrite Scarlett Solo.
Avast antivirus is turned off
Cubase version is 5.

Thanks for any help!

Mike (Spain)

Anyone out there? Has no-one had this problem?