We are currently using DirectRT 2010.122.804.10. We have run several priming studies on these same computers in the past using DirectRT 2006 (and an earlier beta version of DirectRT 2010 - last December), and the presentation times were much more precise (within 10-15 ms). The computers have 1.83GHz processors, 1GB RAM, DirectX 9, and the monitors are set to 1024x768 pixels at 85Hz.
We do get similar variance using the samples that come with DirectRT.
For instance, the sample in "4 - Applications\image priming" presents an image prime for 100ms. During a practice run the log file shows that these primes were presented for an average of 190ms instead of 100ms (SD = 44ms, ranging from 110 to 281ms).
As another example, the sample in "4 - Applications\standard priming" presents a prime word for 10ms. During a practice run, this prime was presented for an average of 114ms instead of 10ms (SD = 27ms, ranging from 49 to 159ms).