There’s never really a bad time to catch flights to the USA, it’s a fabulous place to visit at any time of year, the vast spread of the country means a great deal of variety in climate and seasonal events throughout the year. The West Coast enjoys fine weather year-round, while the Pacific Northwest Coast (Washington, Oregon) has cool summers and mild winters. The Midwest is known for its intense summers and winters and frequent tornadoes and thunderstorms, while Florida enjoys the warmest winters on the eastern coast. Popular tourist spots like New York, Las Vegas and Orlando are most pleasant in spring and autumn as summers are uncomfortably warm. West Coast cities like LA and San Francisco enjoy temperate winters usually not dipping below 10C.
You're not crazy for thinking that a flight price has changed after searching it a few times in your web browser. Based on the cookies in your browser, flight prices do increase when a particular route is repeatedly searched, as the site wants to scare you into booking the flight quickly before prices get even higher. Always search for flights in incognito or private browsing mode to see the lowest prices.