Hi, sometimes when I take my Blazer out of drive/reverse into neutral or park the idle speed shoots up to around 1300 rpm for a few seconds before dropping back down to normal (around 750rpm). I have checked to make sure throttle isn't sticking & that is fine. Any ideas?

 

Sounds like you may have a sluggish or gummed up Idle Control Valve. I'd remove the throttle body, take the valve out and clean the valve and the bore within the TB where the valve seats with carb or intake cleaner. A toothbrush works good for this. Obviously, you may want to check for vacuum leaks first as well too. Try this and see. Be careful not to get any carb cleaner in or near the electrical connections.