I own a 2nd Gen turbo 2 and it's been through a lot of maintenance and it's still a bit buggy.

Whenever I downshift to 3rd gear it starts shaking at 2500rpm.
When it idles, it drops to around 6-700 rpm.
And the turbo has been checked and it's in good condition but it doesn't boost till late or sometimes prematurely.

Please share your experience and your ways of fixing it please?
Thanks in advance.