Do a boost leak test and listen for leaks at throttle body (TB) shaft seals and/or TB gaskets
If the seals are bad you can replace them yourself - search for how to threads - a bit of a PITA or as an alternative send the TB out for rebuild. I sent mine recently to Steve Monroe at Throttlebodys.com
Great service and very reasonable price. The TB gaskets are easy to replace but still also require removal of the TB to get at.