Log-in page just recycles without error message

I’ve had this problem for at least a couple of months, but now it has got much worse.

Sometimes when I try to say write a post, or delete a comment, instead of getting into WordPress automatically from a computer with name and password saved, I get the log-in page.

Putting in the correct entries and hitting log-in, however, only gets the log-in page recycled. (No error message, and the incorrect name or password does get the error message.) The blog itself is not down, so I presume the problem isn’t with the server being down.

Now, it seems that I can’t get into either of my two WordPress installations (one of which is installed as a sub-domain of the other, although separate WP installations) from my home computer (a PC if that matters) at all, although I have for the past couple of days been able to get in from work (a Mac). Although I don’t know if time of day (work time or home time) has anything to do with this.

I’ve cleared the browser cache – having had a vaguely similar problem with Moveable Type that can be solved by this method, but it doesn’t make any difference. I normally use Firefox, but also tried Internet Explorer and had the same problem.

I’m stumped!

All help will be most gratefully received!!

Thanks,
Natalie