Default Slow device - stock factory reflash

hello guys ...
using htc first for a long time now ... unlocked stock jb rooted at&t device outside US ..
since past few days it has become slow due to many apps n stuff i did ....
wanted to reflash the device back to the way it was ...
so my question is ...
1.) where can i get the stock images to reflash ?
2.) i know i will loose the root after flash .. but will the device be locked again ?? and needs to be unlocked again ?
3.) what is the procedure to reflash the device ? (not familiar with htc devices as i was a sony user previously which uses flashtool for all purposes) is there any such tool for htc ?

thanks in advance......