Everytime i make a full stop, my idle goes down to 500rpm then goes back to normal.
it started happening after i got my tune-up.
i check my vacuum lines but i didnt see any leaks.

what might be the problem?

check your Idle Air Control Valve if your car has over 90,000 its very possible your IACV took a ****.
mine just went out and its going to cost 200 bucks for a new one.
hope your problem is elsewhere.
good luck