1998 Merc Sable 3.0L

The heat is warm at best when the **** is turned to hot.
The thermostat is new, i replaced it 3 days ago. The **** that controls the temperature was replaced last year because it went in a full circle. I know it works because turning it to cold will produce colder air flow but when turned to hot the air is only luke warm.

My guess is that there is a restriction in the heater core. If i am correct, how do i go about flushing the heater core myself? The local radiator shop wants $70 to do a heater core flush with no guarentee that it won't cause the core to spring a leak. To replace the heater core they want $350.

Any other ideas that i should look at?