Mack turbo issues

I have a 2007 vision with the 460 aset engine. A while ago the turbo started losing boost when the engine is at idle speed like at a stop sign or around a corner. Then there is no power to take off for 5 seconds until the rpms get up. then it runs great again. It wont do it until it is thoroughly warmed up. After idling for 15 seconds it spools back up. Mack dealer had me replace the turbo actuater solinoid for $2200, air cooler pressure sensor for $460, turbo speed sensor for $280. Nothing helped. When I took the speed sensor out it had oil on it. My mechanic said it should not have oil on it. Replaced turbo thinking the oil seals were bad, that did not help either. Speed sensor on new turbo has oil on it too. What am I missing?