It is rare for a specific inventory fare price to change, let alone for a few hours and then return.

Inventory can change and if a fare class sells out then the fare may change. UA may replenish the inventory, but only the inventory has been purchased.

Watch the inventory class and all will become clear.

While it may be a nice trick to change prices with time of day, the US is more then 3 time zones and people are purchasing from multiple timezones. Airlines don't have the ability to be tricky with prices just for you.