What Could Be Causing This Internet Connection Problem?

Hi folks. I have a problem that has been going on for several weeks now and
it's finally gotten to the point where it's driving me nuts.

I'll give you a few examples of what's happening and hopefully, you can
tell me what could be causing this.

1. I'll go to start or reply to a forum thread and the little thing at the bottom
of the browser that indicates the connection will just do nothing. I have to
hit the little X at the top of the browser to stop it, try again, sometimes
several times, before it catches and the post is made.

2. I'll go to post a new article to my Wordpress blog. Same thing happens.
The publish button will spin and spin and nothing happens. I'll have to hit
the X, sometimes several times after trying the action again and again
until eventually it catches.

3. I'll go to Aweber and try to perform an action. Sometimes it will spin
and other times I'll get a message that the page I'm looking for doesn't
even exist, which I know is absurd altogether.

This problem is happening at almost every site where I have to perform
some kind of saving of data. That's the common action. Also happens now
with DLGuard, especially doing lookups, and this never happened before.

Any clues as to what could be causing this?

Is it my cable modem?
Is it a wire in the house?
Has my service just all of a sudden become crap?
Is it my router?
Is it something on my PC that is preventing me from connecting regularly?

I don't want to call the cable company and have them come out just to
tell me everything is fine unless I can be pretty sure that's where the
problem is. I use Comcast.

After a few weeks of this, I've had enough, even though the problem does
seem to be very intermittent.

Any help you can give or advice you can offer will be appreciated.