Just acquired a 79 300 TD. Power locks never worked. Car dieseled for a minute before finally dying then evolved so it wouldn't stop at all when I removed the key. I plugged one of the vaccum lines and everything's back to normal. Then after a month, it's back to not shutting off at all...is there a good place to start troubleshooting the vaccum system/a likely solution?? THANK YOU, Jared