I know the newer motors (2011+) can run a catless midpipe without any major issues, usually just running rich and/or throwing a code. I'm not sure about the older ones though. I'd either reinstall the cats, or get it tuned since that sounds like the only thing done near the time you started having issues.
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2012 Kona Blue 3.7L V6:
Borla LT headers, O/R h-pipe, Borla S-types, 3.73 gears,
Steeda adjustable UCA w/ mount, BMR LCAs, Airaid CAI,
LCA Relocation brackets, AE driveshaft, GT 80mm TB,
Black 18x9/10 2010 GT500 Wheels w/ Nitto NT555s,
Eibach Sportline Springs/shocks & Koni STR.T Struts