Just a quick question about my st205 with a turbo timer installed, hoping someone can answer for me.

Almost every time I turn the key to switch the car off it's as if she wants to completely turn off even when there is at least 10 seconds on the timer. The revs drop as if the car wants to turn off then go back up to idle until the timer is finished then turns off.

Any ideas as to why this might be happening? Thinking it could be a faulty turbo timer.

Sounds like it to me. I'd get rid of it - they are unnecessary with modern oils.