David,
It's good to hear is someone hitting the ball straight!
Your swing should bottom out at the ball socket of the left shoulder (right handed). This is the pivot point. Each club is designed to contact the ball then the ground at a different point. There are 2 schools of thought on ball position.
1. Keep the same stance and move the ball position.
2. Keep the same ball position and move the right foot in to narrow the stance for each club.
I prefer to keep the ball in the same position and take a narrower stance. Some of my students do half and half and some move. The hard rule is take a practice swing and take a divot. Place the front of the ball at the begining of the divot. It sounds like a ball position problem.
There could be some shallowing in your swing but that is hard to tell. If you want to submit a 4 -5 iron video, it would be easier to tell.
GolfBald