I had a similar problem with the stock manifolds melting the plastic portion of the same line as it snakes down between the firewall and engine.

After splicing the line a couple of times (once at 4am in the middle of a mud hole) and re-routing it each time I finally repaced the entire line with a higher temp line and re-routed it behind the heat shield on the passenger side.

I recently installed headers and have had no problems to date with heat, but I will definately check it out for wear on the solid lines.

Thanks for the heads up!