JLG G1255A miss under load

Hey guys, I bought a used 2009 JLG G1255A under driving load I hear and feel a miss, you can not hear it from the engine side of unit but can hear it clearly from the drivers seat. I called to get someone to come look at the machine, the asked for the s/n of engine and here is where the fun starts... The unit was rebuilt by Pacific Power Products and has only there tag on it. They list it as engine family BPKXL04.4NJ1 and states year rebuilt 2012 1104D-E44TA. that info brings up perkins...

JLG G1255A miss under load

JLG G1255A miss under load