There is a device that controls the second set of valves and these give trouble with vac leaks etc.
As you probably know it works as a 12 valve engine until a certain point is reached then all 24 start working noit too sure on what goes wrong but think it is normaly easy cheap fix.
Although this is an ace site FMOC might have better answers if you have no joy here but still worth asking here first.
tabetha