Hello... well a minute or two between boots is not a extreme problem in my eyes... sometimes it takes time to drain the voltage, resetting the 1's and 0's, back to start logic.
left in a CAR or trunk... Ya that sounds like vibration, extreme temperature swings?... and a re-flow B /... there are quite a few utubes on the subject.
Cracked solder joints are visible defects with a magnifying glass... the Bad solder joints will be a dull shine, no shine or a Visible crack OR obvious discoloring of the Circuit board from heat... another way to locate them would be slightly pressing on chips or spots on the MB, and repeating BOOT tries... too narrow down the location.
I use the above method with a magnifying glass and solder iron to the questionable joints... I have never re-flowed with a oven method... and makes sense for a quicker repair solution for this type of Solder, surface mount and smaller solder connections.