'01 Altima misfiring under load

[Jiinx]

Hi Guys.
I have a 01 Altima that is lugging and misfiring under load. When the car is cold, no symptoms. When it is at operating temperature it lugs out when you step on the accelerator. The only code it throws at me is an engine misfire.

I also noticed that when the misfiring happens the Temp gauge is going from the middle of its range to cold.
When I back off of the accelerator, engine runs fine and the gauge goes back to "normal". The front Flex pipe was changed in the car recently. It hasn't been a problem before then and I don't see any damage on any of the wiring in the area.

Anyone have any ideas?