Problem with Remote Desktop Connection

I have an intermittent problem connecting to my office desktop computer,
which is also running Vista. About half the time I can connect without any
problem, and in fact had been connecting with no problem for several months.
Recently, though, after successfully connecting via VPN, I get a message that
says "This conmputer can't connect to the remote computer. Try connecting
again. If the problem continues, contact the owner of the remote computer or
yur network administrator." This can go on for several days, then without
making any changes in either computer, I can connect. Then after only one or
two days, the connection fails again. I have FIOS, and it is generally a
reliable Internet connection, and my home computer runs Vista Ultimate.

Please help!
Martin