Goals from Luciano Becchio, Alex Bruce and a Davide Somma brace cancelled out an early Jon Parkin strike and the Whites looked to be heading for a comfortable win.
But two more Parkin strikes, a Callum Davidson penalty plus a goal apiece for Keith Treacy and Iain Hume completed North End's stunning comeback.
Grayson said: "I am hugely disappointed. Football never surprises you, you are going well and suddenly it kicks you where it hurts again.
"When we were 4-1 up, we needed to be more professional but we gave them a chance to get back into the game.
"There were far too many errors - both individually and collectively. We made stupid mistakes and it cost us. To lose 6-4 at home is unbelievable really, and embarrassing."
Source: PA
Source: PA