I'll refrain from making my usual "You get what you pay for" remarks- especially with my dear friends at Intamin :) - oh wait, I guess I didn't.
The trains on Screamin' have done quite well, for now, but we'll see how long that lasts.
The launch system (not the upper slow crawl system) was built without proper cooling for the ambient temperatures that occur (whether or not the ride is in the sun).
Add to this, Disney's desire to launch rather frequently, and you've got a recipe for overheat - don't worry all you sissies and worrywarts, it's nothing dangerous- just annoying as it shuts the ride down. They have tried stopgap methods, but so far they've only been marginally effective.