Under exposure at high shutter speeds: reciprocity failure?

[scsambrook]

I find that on both my digital cameras I get 'underexposed' images when shooting at wide apertures in bright lighting conditions - typical exposures 1/2000th or 1/4000th. At first I assumed this was a functin of reduced contrast with the lenses I was using, but a little experimentation with higher ISO settings and smaller apertures has satisfied me that underexposure is actually the problem.

Those of use who grew up with film will be familiar with reciprocity failure at very short (or indeed very long) exposure times - but is this possible with digital imaging?

The alternative explanation could be shutter inaccuracy, but usually focal-plane shutters have tended to over-expose at their highest speeds.

Has anyone had similar experiences?