holes in resolving?

[Len Conrad]

I've had a few instances where I could not browse to web sites, using a DNS 
of a a neighboring company.  Now I run my own BIND 8.1 and find that there 
are still domains I can't find episodically. eg, this afternoon, I couldn't 
nslookup or ping compaq.com or cisco.com, but when I used a another DNS, I 
could. In the evening, both cisco and compaq were findable using my DNS.

Is this a known phenom?  explanations?

Thanks,
Len