Hmm.... Id really question your IPR valve. Too high of injection pressure is were you get the lope but the 1211 code is a low pressure code..... Kinda a stumper. Should find were that leak is from. if its the pump rear case then its deffinantly time for a pump.
Unplugging the ICP put you into limp mode but if that clear up the dying issue it tells something at least. Hows the plug/wiring at the IPR valve?

Terrimator T500 is a real good upgrade anyway. Hit up Jody at Dp Tuners. He gets them at wholesale so theyre slightly cheaper thru him.

 

My truck is doing the same thing.... you ever get the problem fixed?
Thanks...