check the turbo for shaft play, i think you have a seal and bearing wear problem
Sounds to me like your turbo is on its way out... At least the seal on the exhaust side is anyway.
How does the tailpipe look? Is the soot dry and powdery or wet and sticky? Any difference in smoke color or smell (especially when cold)? Turbos are a Craps shoot when you think about it. Some will last forever, while others won't make it to the first oil change.
Its time for a rebuilt turbo. Be sure they balance it. Depending on usage, you may be stretching the oil change intervals a bit too much. Also, change the oil when you change the turbo.