Your actual alignment specs, since those are the recommended specs you posted and not your actual numbers since no shop would tell you your caster is "3° to 5°". Also, if you want to be doing 120+ you'll want more aggressive alignment numbers than what's there.
But we're getting somewhere. You changed all that suspension out and still have the same issue? So, that would indicate it's probably not the suspension- so aero, or something else...
Holy crap Batman. See, this is why we asked about how your car is set up.
You're running "S" rated BFG hockey pucks at 120 mph? Those tires are only rated for 112 mph. You think you're going to get good performance out of those at 120? Not a chance. Those tires probably wouldn't even make it to 150, and there's nothing like an overheated tire blow out at 120+ to make life really entertaining.
I'm sure you still have other issues, but those tires for those speeds are a really, really bad idea.
The Randy's spoiler will do something, like I said, any air it keeps from going under the car is a win. But when you call and order it, ask if he still makes the "tall" version. It would be more beneficial for you.
That one is a copy of the DC front spoiler. It should work, but you're doing to need to add some bracing because all it appears to be is a thin layer of fiberglass. You'll need to back it up to keep it from just folding over at 100+.
