incorrect results when using ttmath:UInt<4>

I used ttmath:UInt<4> type in an application. Only addition and subtraction operations are used on the types. I complied the code under 64 bit Linux with g++ 6.2.0 (-O3 optimization). When all variables fit within memory, the results are always correct. When the allocation is a bit larger than physical memory and some swap storage is used, the results are incorrect and different across different runs. Could someone help troubleshoot the problem? Thanks.