This 76 mm diameter idler pulley can be used to correct belt length when changing supercharger pulleys or harmonic balancer sizes on the C7 Z06 LT4, C6 ZR1 LS9, Cadillac CTS - V gen 2 LSA and gen 3 LT4 or Camaro ZL1 gen 5 LSA and gen 6 LT4 supercharged engine. (weaponxmotorsports.com)
A compression test would be the natural first step for checking this possibility, It's also possible that you have inconsistent belt tension as the engine rotates due to a bad idler pulley or bad bearings in the water pump, alternator, AC compressor, power steering pump, etc.. (mechanics.stackexchange.com)
He has serviced the car in preparation for sale, with work including a new idler pulley, A / C and alternator belts, oil VANOS line, oil filter gasket, valve cover gasket, oil / filter change, and more. (bringatrailer.com)