Dies when rpms come down

sounds like your VSS is taking a poo poo on you.

The VSS piggy backs onto the speedo cable, and there is a harness plug that goes to the VSS. If that plug was never connected, then the speedo not working should not affect anything. If you do have it plugged in, then I would say that the speedo not working could absolutely be something to look at. I would also not assume that the IAC you swapped on was good, unless it was brand new. If the IAC is not working, the ECU would not be able to maintain idle when loads are place on the engine. I would also look to see if you have coking around the throttlebody butterfly, or if the stop is adjusted closed. In either case, the engine won't get enough air to idle, so the iac has to compensate and open more to maintain idle. If the IAC is faulty, it won't be able to supply enough air flow, and your engine dies. As always, YMMV. Good luck, I know how irritating idle issues are.