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Is it because 757/767 uses FSUIPC?

No. This is explained better in the Support Knowledge Base, than I can really do, but I'll try anyway. Because of the complex coding in the FMC of the Captain Sim 757 and 767, increasing the sim rate above normal (x2, x4, etc..) may cause the coding to get behind what needs to be done or what IS being done. This can cause it to "go crazy", as you put it.

I have problems with many of my airliners with complex FMC's and increased sim rates. These include "Other" 747-400/8i, MD-11 (never tried it in the NGX yet) and most others at that level of complexity in the FMC. So I fly at normal sim rate only (x1) to avoid crazy aircraft behavior. I have flown 16+ hour flights in the CS767 at normal sim rate of x1.