RPM drop when shifting from P to D

[Mike The Canadian]

Just curious on what could cause this..

My idle is set to around 750-800 rpm in my tune..
Ever since getting the heads done when I shift from P to D it drops to about 400-500 and then raises back up.

What could cause this?

I fixed a tonne of vac leaks (anyone who knows my car will laugh) last time I was at home. It was better but still does it.

I almost want the vac leaks back, stiff tranny mount + idle at set speed instead of above = a lot of vibes!!