you could try switching the tire with the one on the other wheel, just to see if the same thing happens. If it does, it's the combination of rim and tube that's the problem. I guess you could have a tube with a wonky valve mounting, or a rim that's giving way, but I'd think you'd notice that otherwise.

If the problem goes away, it's the tire that's the problem (or the tire with that particular rim). I've only experienced this with a tire bead about to give up the ghost.