If it's heat related, then you need to bleed out an air bubble. But if you're looking to avoid that here's a quick diagnostic to rule air bubbles in or out.

Put your bike in the sun for a while, and see if it's the sun or that you're riding that's the problem. If it's the sun, the brakes will rub without weight on the bike, but if it doesn't than something is moving under load.

The most likely load related cause is sloppy wheel bearings, axle play, or possibly frame deflection (but not likely disc warpage). You can confirm this by seeing if you have problems riding where the sun isn't heating the lines. Find a place to ride it indoors, or during the cooler evening, and see if you have problems or not.

Either way, your first step should be to try to isolate the issue as heat or weight related, then move from there.

If it isn