Datalog uneven interval timestamp

[npf101]

I have just started looking at the Datalog function having been using the Megajolt for a couple of years now without any issues. I did a number of runs and noticed that the timestamp interval seems to be random! With the sample interval set at 250ms ( I can't seem to select 100ms) the data in the created csv file has the time stamp interval varying from anything up to 1.5s down to -0.1 seconds. Has any one else seen this and is it likely to be a function of my laptop spec?
The attached file shows some examples of the varying time step.