Interent connection acting flaky & unreliable

[Petef56]

Very odd problem, difficult to troubleshoot on brand new Dell
desktop PC with Vista Home Premium and Comcast ISP.
No router. PC is wired direct to Comcast modem.
McAfee Internet Security SW.

Problem:

Interent connection is not reliable. IE somtimes reports "page
cannot be displayed" at any random website or webpage, but
if you click refresh, it will display the page. IE also seems a bit
slow upon request for URL or click a link, there is somtimes a
few seconds delay until the page is loaded & diplayed. Another
symptom, is that while installing software, if the software
attempts to DL an update, the installer somtimes reports it
can't locate he connection. Click Try Again, and it finds the
connection and works properly.

Basically, the Interent connection works fine at times but
sometimes, one minute it's normal and the next minute
it acts flaky.


Troubleshooting:

Tested speed of connection using the Verizon speed test and
one time it reported 650 KBPS speed and 2 other times it
reported about 2200 KBPS speed. Suspect that high usage
of the Comcast service by other people in the neighborhood
could be causing the problems, but it's highly unusual to
get "page cannot be displayed" errors in IE.

Tried shutting off the McAfee firewall, but problem still occurs.

Tried restoring Advanced setting in IE to defaults and also did
the "Restore IE settings" in the Advanced tab too, but problem
still exists.

I'm having the person/owner of this computer monitor the
problem more closely to tell me whether the problem is
better or worse on weekends, or off hours when most other
people might be sleeping.

I don't suspect malware becasue the people report that it
never worked right from the start.

Any ideas what this problem could be?

---pete---