Have a 97 ZR2 keeps thowing p0507 code. Replaced IAC,TPS,PVC.Thermostat,VSS,temp sensor,cleaned MAF, cleaned air filter[K&N] checked for vac leaks[manifold/replaced lines. On start-up idle is fine about 900-1000 after heats up or run on freeway stays at 1200. got 180,000 on it but need to get emmisions done for license re-new. systems re-set every time code is erased. Help!