With what you wrote, I'm sure you realize it could be anything. Let'* go through the basics and work our way up.
Those coils are close enough for what is needed. I'm going to assume with all the work you've recently done that the plugs and wires are fresh. The #4 and #6 plug wires should be checked to ensure they aren't touching the O2 wiring.
How many miles on the car? Any idea if the upper intake has ever been changed? (Has date codes on top of it) Any idea if the fuel pressure is good? Please pull the vac line off the regulator and double check for any fuel in it.
00 Regal GS: All kinds of cool stuff.