problems with interpolation function

I'm using measured data on rain events with 15,000 entries. I'm now trying to implement the data into comsol as a function of time. I have found that using the interpolation function works when using less than 300-500 data entries. When Using more than 500 (all settings equal) the data looks corrupt when plotting the function.

I have attached a .jpg showing the first 200 hours of the interpolation. The first one is using 200 entries and the other one is using the whole dataset (15000 entries). Any idea why they differ?

Jonas