Fix a problem with multithreads in a vb.net project.

hi, I have a vb.net project. the program is a SMTP tester that you can load a list of smtps and the program start to check using multi-threads for make the check process more faster. everything work fine, exept when I try with a big a list of smtps the program start fine but in half of check process everything go very slow, like if some threads are frozen or busy. I need fix it for that the threads continue with same speed like when program start. I attached a screenshot. If you think that you want fix it I can send you the source code. thanks