tire/brake/suspension

iono which one it is

but here is the situation

when i start to go, everything is smooth
BUT
when i reach 40-45 mph...... the car starts to vibrate
if i speed up, it stops
if i slow down, it stops
it is only in that speed variable

so what can cause this

flat spot in tire?
off balance tire?
warped brakes?
bent suspension?

please help
i just got new tires and rotors about 2 months ago

the tires are still under warranty but the people claim by my description that it is not a tire problem, so wtf

im about to take it in, but i don't want to pay $125 for a damn inspection

If you've flat-spotted the tires, it's a possibility. It's also possibly an out of balance tire. Both would typically affect a much wider speed range, but I sure wouldn't say it's impossible that it's either of those.

It's not the brakes, or the problem would only be there when you were on the brake.

Check the steering/suspension. I have seen many loose tie rod ends that cause a vibration in the steering wheel in a very narrow window of speed.