K5 interval shooting - exposure accuracy.

Hi, I have a K5 since July 2012 and taken just over 5,000 shots. However, my love affair with it is under pressure.

Using the interval timer has been no problem until now. Yesterday I had camera set up in cloudless conditions taking shots of birds feeding. On `In camera playback` the shots alternate light - dark - light, although the information bar at bottom of LCD screen shows 1/640sec - F6.3 - ISO 400. Any suggestions please, as to what is wrong, do you know of a simple fix or do I need to return the camera to Pentax ?

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Regards - Richard
Happy guy with a K5
N.B. All my Images, even the bad ones, are Protected by Copyright