Hey, it'* great to have you here. Even better with someone from Russia! Amazing that you have the US car hobby. Must be difficult at times.
Your dealer is right. There is a bypass valve (or wastegate vacuum valve). It only costs about $20 US. Are you sure all the vacuum lines are connected?
The part number for the valve is 12353309 and costs $12.98 US.
In case you need to replace the harness for the valve and FPR, the part number for it is 24502303 and it costs $8.46 US.
These prices are from www.gmpartsdirect.com
and don't include shipping cost or a rubles conversion! Hope this information helps. Your valve could be sticking, or you could have a vacuum leak. Fuddyduddy may have more to add here, or may correct me.