Unexpected Result When Using Sinc Interpolation
Since Sinc based Interpolation requires you to know the data at any point it can not be done.

You might do a Truncated Sinc Interpolation.
The artifacts you're seeing can be caused by a kernel which is too short or the parameters aren't good.

In order to create a good Sinc kernel you need to know things about the Band Width of the signal and the Sampling Rate, did you took those into account?