I have a problem with my 1990 Suburban. It has a newer 350 in it with 20k. When I go to start it the thing starts up fine, no throttle needed or anything, just turn the key. It'll run like normal for 30-45 seconds then it just shuts off. Doesn't seem like it dies, but rather shuts off, as if you switched off the key. It leads me to believe it may be an electrical problem but I have no idea. When I start it up the check engine light doesn't come on, so I don't know if it's throwin a code or not. Any suggestions?