Car only overheats when AC on?

In my experience when a motor slowly overheats with no obvious cause it's a clogged or dirty radiator. Normally the radiator has deposits on the inside that are hindering the heat exchanging process.

Sometimes it helps to flush out the radiator and cooling system. However with the cost of labor that often it's cheaper to just put a new radiator on it.

As I stated in my original reply, "Sometimes it helps to flush out the radiator and cooling system". Sometimes is there because flushing doesn't always remove the deposits inside the radiator.

The real problem is that the home based flush products are not all that strong. A professional radiator shop would remove the end caps and hot tank the radiator. Followed by a rod and core. However the Mustang uses a plastic end cap that is crimped on. It's not practical to pay a professional radiator shop to clean a radiator such as this.

If you look inside the radiator openings and see the inside covered with white deposits, that's the bad stuff.

Make sure that the motor's health is good (head gasket not leaking) and the rest of the cooling system is in tip top shape. Confirm that the cooling system is holding pressure.

In the interest of being through rule out a clogged exhaust system.

When everything else has been ruled out, what is left is a radiator that is clogged on the inside.