Rebuild the flippers. More than likely a melted coil sleeve and a mis adjusted EOS switch.

First I'd determine if it's a mechanical (binding) issue or an electrical issue.

When the flipper is stuck in up postion, turn the machine off immediately. What happens? If the flipper falls back down, it's electrical. If it stays up, it's mechanical.

Or take the glass off and when it happens, push down on the flipper. Fall back down easily? Mechanical issue. Pushing back at you? Electrical.

If it's mechanical, follow Shane's advice (although often just getting in there and realigning things and tightening everything back up good and tight will be a fix)

If it's electrical, your next step depends on the game you are dealing with.

One easy thing to check is if there is some vertical, up down play in the flipper shaft. If your flipper is tight to the flipper bushing then it will start to heat up and bind. It should be just loose enough to lift the flipper bat up a tiny amount. Check for this and then tighten everything up.

While everything is open check to see if the plunger and coil stop are deformed in any way. Could be a mushroomed plunger head dragging inside the coil sleeve.