orion ghia efi HELP !!!!! please!!!

[v6vec]

I`m having a problem with my old `onion` ghia as it seems fine to drive for about 10 minutes then loses some of its power whilst driving. i turn the engine off and back on and it goes fine for about another 10 minutes. i`ve given it a full service, (oil and fuel filters, plugs and leads, fresh fuel and oil), and it hasn`t changed! Any advice would be greatly appreciated. thanks, andy.