Starts and idles fine, when I engage D or R it kills. I can sometimes slowly engage and get it to stay in drive and idling or even running but then it eventually kills off. I noticed what seems like the sound of a relay switch and the check engine light comes on- and a slight dip in the voltage gauge. Wondering if that’s connected and if it’s an electrical issue, or if that’s coming on because of what the core issue is. Replaced the IAC valve- didn’t seem to make a difference. Any ideas?

 

If your vehicle has the original ignition coil on it, they had internal problems with them.
Might be a good move to replace that since you have no way to prove it may or may not be defective.
Not an expensive item and easy to replace. Let us know if that was the problem cause, thanks.