My 87 5.0 starts up but only runs for a few minutes before dying. After it dies, it will not immediatelly restart. After performing a compression test, it was discovered that cylinders 1 & 2 only tested at 15 psi each, and cylinder 3 only tested at 60 psi. Could this be caused by the head gasket or something else like a valve or rings? The block has less than 5k miles on it.