79 450 sl runs erratic at idle and will die

[Cruiserlodown]

could use some ideas here. My 450 will start and run at high idle nicely but after a minute it starts die-ing out. Cannot even try to give it any throttle because it will die right down and start skipping and backfire-ing. and evenually stall out. Will start right back up and hi rev smoothly but after a minute starts losing revs and skips then stalls .Will start right back up again