Tell us your specs, what game, have you googled, do others have this problem, have your updated drivers, have you tried turning audio off and/or lowering settings?
As I think was said, it could be complex to pinpoint to answer but generally it will be do to with loading resources, the relationship between gpu, cpu and system memory can be complex. Allocating and deallocating memory blocks, the memory manager(os) has to constantly search through all memory to find a suitable block of memory if not it resorts to virtual memory(paging) where the hd is used instead of system memory, this is slower and could result in the symptoms described, but it could be a line or two of badly implemented code, some overly complex shader trying to run on the gpu, or some overly large audio file trying to be fetched by the gpu from memory. Generally it's probably some bad coding doing it.
As I think was said, it could be complex to pinpoint to answer but generally it will be do to with loading resources, the relationship between gpu, cpu and system memory can be complex. Allocating and deallocating memory blocks, the memory manager(os) has to constantly search through all memory to find a suitable block of memory if not it resorts to virtual memory(paging) where the hd is used instead of system memory, this is slower and could result in the symptoms described, but it could be a line or two of badly implemented code, some overly complex shader trying to run on the gpu, or some overly large audio file trying to be fetched by the gpu from memory. Generally it's probably some bad coding doing it.