I've managed to export and import the task and it does seem to be behaving better with a higher priority, although the subsequent rfutil command is called inside the .bat as;
Code:
%COMSPEC% /C _rfutil.exe %db% -C roll forward -B 5000 -a %ai%.ai%ai-num%
and using ProcExp it's showing as the rfutil command running at a 'normal' priority.
I'll have to test an alternate way to call _rfutil so i can set the priority.
Thanks for you help guys.