Sounds like your coil packs mate,they will most likely have hairline cracks in em from being under the cover and getting hot. pull em out, wrap some fire retardent electrical tape round em n see if that helps. if so time to look at some spit fire packs!
do a search for more info, has been heaps of discussion diagnosing problems that ends up bein coil packs!~
if its not that, put it on dyno and see if its leaning out, could be fuel pump playin up.
cheerscheers!
brett