1991 camaro rs 3.1 trouble

i have a 1991 camaro v6 3.1 rs i have had to put a new computer in it and it will run now but it runs rough but if i take the spark plugs out it will run fine for about 5 minutes but then will start running rough and then eventully die iv been told it has either a blown head gasket or a blown intake manifold how can i tell which one? i really need some help so i can start working on it thanks.