PWM stopped when using Ethernet+SD

Hi,

can you please help to explain this behavior?

Arduino Mega with Ethernet shield with SD attached, programmed with merged examples of SdFat / SDWebBrowserView and Digital / BlinkWithoutDelay. LED controled using PWM works as expected on each of available 12 PWM pins (10 and 4 dedicated to Ethernet and SD) until the moment I click on link in browser to display a file contents. It takes several seconds to read all file contents and during this period PWM simply stops at current level and continues after browser finishes reading all file contents.

Why does PWM stop? Is this related to SPI bus, interupts or something else?

Thanks.

BUMP

If no one can explain this, can somebody verify/confirm this is not my specific issue but a general one, please? Thanks.

As far as I have read this forum I never came accros a similar problem. Could you share your code so we can have a look at it if there are any possible causes?