Hi frogo,
I don't think you are having any problems as such, but it may be a feature implemented on the forum at some point last year when the forum was under a spam attack of sorts.
To prevent people registering on the forum just to then rapidly send PMs a change was made. The change was to prevent members from sending PMs to each other immediately after another. Yes, there may be users who don't wish to abuse the PM function of the forum, but the forum software isn't smart enough at this stage to figure out if your intentions are good or not!

So everyone now has to wait a little while before sending another PM. Something to note though is that the forum time is currently off a bit. It is about 20 minutes fast, so you have to account for that when it tells you how long you have to wait to send another PM.