99 ram 1500 4x4

plenum just replaced in june

The trucks been acting funny lately...not speeding up anymore past 4000 rpms...
Thought it was a tranny

but then today, i was driving down the road, and engined just revved and check engine like flashed.
checked teh codes, multiple/random cylinder misfires. Checked twice and all 8 were misfiring at some point.

Going to get the cat pressure tested tomrrow if it'll run. cant get it above 2500 RPMs or itll just stop.

Any other ideas?

 

Check the basics...tune-up, fuel pressure, spark plug wires, etc