You are right, there is a problem. The problem is caused by a bug in the FFT VI in LabVIEW 6.1. It does not compute correctly when the number of input samples is a "big" prime number (in your case 59981 samples).
The work around is to add a "dummy" sample at the end of your time signal before you perform the FFT and remove that last sample at the output of the IFFT.
