Sudden latency - please help

My system consists of a mother keyboard sending midi into cubase via a profire audio interface, playing an external (motif) sound module.
My problem is that out of the blue a latency issue came up (after over a year without a problem, so probably I messed something up and it is a simple fix).
What happens is that I play the keyboard and everything is fine (no latency at all) then suddenly (usually after a minute or two) extreme latency suddenly appears, which is such a delay that the keyboard is unplayable. I have noticed that there appears to be a midi signal/pulse of some sort passing which initiates this problem (there is a visual indication on the transport bar).
More info : I notice that there is no latency when playing back a recorded midi track from cubase (even when the latency has kicked in for direct playing). This obviously means that recording another track to add to the existing one is impossible as the timing is off and the keyboard unplayable.

I dont really understand this at all, as to my mind latency should not even be relevant, as it is a midi pathway throughout, with no conversion to audio except within the motif sound module. (ie in simple terms just a (midi) line connection through the system of my mother keyboard to the motif sound module.
Would really appreciate anyone’s input.

The first thing to do would be to check the obvious.

Is the correct driver still selected and if so has the sound cards buffer settings changed?