P0301 code came on and the truck is lurching a low RPM on 2004 GMC Sierra 2500 HD

We bought a GMC sierra 2500HD a montha go, low miles. The check engin light has came on a few times and disappeared after a day or so. The we got off of it was P0301. I changed the spark plugs with Irriduim plugs and the code came back on this morning, but I noticed it is lurching when I let off the throttle this time. Any Ideas?

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1 answer 3 comments
POSSIBLE COIL PAK OR WIRE ISSUE WHICH IS COMMON
how does the cylinder numbering go on the new GMC's?
d/s is 1 2 3 4 p/s 5 6 7 8


So I tested the plug wires and coil this weekend. All were functioning fine with the check engine light on. I notice a few other things this weekend, the rpm meter and the speedometer are pretty jerky and the light only seems to come on when I am pushing the engine (ie: when I pass or pulled a hill or haul a load). There have been a lot of modifications done to the intake and exhaust, If they reprogrammed the computer would that cause these issues?

Did some reasearchand it seems the was a recall one the instrument cluster but I bought the truck a few months after the deadline.