I have looked through the logs at the lag spikes.
Apart from the normal ones that occur at save time (~30 minutes past the hour) and at backup time (every 6th hour) there do appear to have been an abnormal spike of lags for a few specific players.
The server itself did not detect any lags, so it is not on the physical machine.
More than one player was getting lag but this appeared to move with time.
Not all the players online were getting lag, which indicated that it was not the server centre.
The last time I investigated this type of incident of clusters of lag, there was some event going on in the US (like 4th July or something similar) and the internet was flooded with messages flying around.
There are also incidents where a particular node on the internet has a problem and anyone going through that node gets affected. The closer the node is to the physical server then the greater the chance it will affect more players.
A possible solution is to reset your modem and it will reselect new routes to the internet for you. Depending on where the problem is, this may solve the issue or have no noticable effect.