First, there are no such things a "Slipped Belts". There are "separated belts" and sometimes those are incorrectly called "slipped".
So rub a GLOVED hand over the tread circumference. The glove is to protect your hand from possible wires sticking out.
You are looking for a bulge. If you find one, drive SLOWLY to the tire shop and replace that tire.
But since the problem goes away, I'm voting for flat spotting where a tire takes a set after sitting in one spot and gradually the flat spot works its way out (due to heat). Try using more inflation pressure (say +5 psi) and see if that changes things. If it does, then it is likely a flat spotting phenomenon.
I found this on line, can't vouch.