Putty psftp login problem

I have used Putty for a number of years now for ssh logins to my Debian
and Ubuntu machines, many, many thanks for a brilliant set of programs
and service.

For about eight months or so now I've hit an erratic problem. It's
happened on two different Windoze machines, both Intel (Dell laptops)
one running 2k with all patches to about four months back and now an XP
machine (build 2600.xpsp.050622-1524 sp2) with direct or wireless local
connections and either inside my firewall or outside it. Machine I'm
trying to psftp into is an Ubuntu machine now but it happened when it
was Debian too. I've been through several versions of Putty, now.
0.58. Putty never shows this, only psftp. What happens is that I give
the username and then nothing happens, the sessions hangs and generally
eventually kills itself. I can see nothing useful in the server logs
(but may be looking in wrong places). Happens >50% of time I try to
log in, if I log in with Putty at same time, no problem with that. No
logic I can see in when it will just work (fifth attempt today worked
fine but first four failed).

Any thoughts/suggestions? Assume pretty competent non-computer-pro,
much happier and more savvy with Linux than Windoze but no choice in
having to use M$ hence the problem!