It may be that your phone just doesn't have enough free resources to update the screen. I.e. it has a few open browsers with flash left overnight, it may take a few extra seconds (as in 15) before the screen gets updated.

Myself, I've experienced that the blue led starts blinking at least five seconds before the phone starts vibrating/playing the ring sound, and then the display may not turn on for another handful of seconds. In some cases it's gone to voicemail before the phone registers my attempt to press the accept button. So, the next time this happens to you, you might want to check if something is using the CPU.

This is a concequence of having one of the very rare phones with true multitasking, you get the first phone that complains about not being multicore.