Here are your parts
Parts for DV448AEP/XAA - AppliancePartsPros.com
You will have to choose the correct unit from the Matching Model Numbers section.

Has this unit ever worked OK with this ducting set up?
Do you have the wiring diagram? I do not have one.

Unplug the unit and both wires from the heating coil.
Check the coil, most coils are in the 8 to 12 ohm range.
Then check from each side of the coil to it's case. Both should be infinite ohms. If not the heater is grounded and should be replaced. this can cause it to run unregulated.

Check that the blower wheel is properly attached to the motor and not slipping.

Check that the blower wheel is clean.

[COLOR="Blue"]Your thoughts?[/COLOR]
This is a pain.
The thermistor is worth a try.

Did you check the heating element with a meter?

So this is a new location, correct?
Did this unit ever work OK for you or is it new to you?
If just new to you are you sure that it worked OK at the previous location?

Check the lint filter. Sometimes it can become coated with fabric softener residue and has to be cleaned with soap and water.

Check all seals in the unit. Before the heater it is actually under a vacuum sucking in air. So if there a bad seal etc. it will pull in cold air reducing the temperature at the blower so the thermistor sees this and keeps the heater on too long.

Since everything seems to be OK after the thermistor all that is left is the control board which may be intermittently holding the heating relay closed.