Frankie,
With a 1 year old rodding & somewhat fresh stat, I'd start to suspect water pump vane cavitation. B4 opening that up, however, I'd double check radiator for bugs between ac condenser & radiator; lotsa guys marvel at the accumulation there when all else looks squaky clean. If that's okie-dokie, its gotta be somewhere else. Pinhole leak would be next to look for (allows coolant to flash off & escape w/out holding system pressure); check all hoses & use a coolant pressure guage to check if its holding pressure. then coolant flow is next up- meaning try a new stat (its cheap, fast & then go to water pump. Fan clutch could be bad, but I wouldn't expect enough heating to require the clutch at idle unless something else is wrong.
Good luck.
\'95 P30 Motorhome 16,500gvwr, 26,500mi<br />New Ipump @ 25k, boost, EGT, homemade remote FSD cooler