Frigidaire Dryer not drying clothes properly

Dryer not drying clothes like it used to. It's about 2 years old and recently it takes about 50% to 100% more time for a load to dry. Here's what I've done and discovered:
1. checked complete exhaust ducting, all the way from inside the dryer, past the fan, through the internal duct to the connection in the wall. All clear, and very little lint was found. (dryer is plugged into a 240V circuit)
2. noticed that some heat is being produced, so either the heat element is okay, or element might be partially working. Not sure how to tell
3. checked the following guide to try to produce an error code. I got an E68 (button stuck), but suspect that's a red herring because all the functions seem to work (see Function Test Sequence in the following link), although I suppose it may point to a bad electronic control panel. http://manuals.frigidaire.com/prodinfo_pdf/Webster/134682200ts_wd_EFS.pdf
4. ran the second function test in the above guide (Drive motor runs; heat source is on. Drying LED is lit “H” and the control thermistor reading will toggle back and forth in the display) and the reading started at 88 and within about 5 minutes went up to 130; then started dropping until 121, then started to rise again. Could this mean a problematic thermistor control?
5. the wiring diagram in the above guide indicates there is a “High Limit Thermostat” connected next to the heating element. Is this a potential problem
6. the wiring diagram also shows a Thermal Limiter and Heater Relay. Could these maybe be malfunctioning?

Is there any way for me to determine which of the parts (heat element, control thermistor, thermal limiter, heater relay, electronic control panel) might be the problem? Or is it better to call a repair person?