First of all, that's a rather old version of the simulator. When using IC5141, you should really use spectre from the MMSIM stream (e.g. MMSIM10.1 or MMSIM11.1) because spectre in IC5141 was superseded by spectre in MMSIM in late 2004, and has had no significant changes - over the last few years it has had no changes at all - even in the latest hotfix of IC5141 it's from December 2008.
I simulated using a slightly later version of spectre (126.96.36.199508 - so 4 months newer), and plotted the results in the latest version of IC5141 ( 188.8.131.520.6.151 - which dates from November 2011), and I see the same results as I did in IC615/MMSIM11.1. See the attached plot (from wavescan) - I get the same results with the older AWD calculator/waveform tool.
As I asked before, how did you plot the FFT results? You mentioned in another post that you were generating a csv (how?) and so I'm wondering if you were doing the FFT in some other tool (Matlab, maybe?)