1990 Chevy Lumina cracked manifold

A cracked intake will definitely affect how the engine runs but it should not just shut it down. It should only idle rough in park or at stop signs. It scews up the readings recieved by the computer and the computer fights to keep the fuel mixture correct from air coming in where it is not supposed to.

When you are at full speed the throttle body is open more and reduceing the affect the crack causes. The only way the crack could cause the engine to fail at highway speeds is if the engine heat expands the crack to over ride the computers ability to cope with the problem.

Have you tested to see if the crack is actually open?
Spray some carb cleaner in the vicinity of the crack and see if the engine responds in any way like a rougher idle or faster idle. If it does then the crack is open. It is just possible that you are simply seeing a blemish in the casting that looks like a crack.