Original owner of Kenmore 90 series; is taking way too long to dry even on high heat. I have replaced the fixed thermostat (3387134), the thermal fuse (3390710), and both valve coils (279834). Also replaced drum seals a few years ago. The flame comes on and stays on and looks OK. Vent is solid line and very short, only about 3 feet and totally clear of lint. Blower push air through vent fine. Just not hot enough and takes 2 times longer to dry with half loads only. What would the next step be? Thanks!

Thanks for the insight PNWDrew.
I used an Infrared Thermometer to check the temps. With the drum spinning and the dryer on High I got a reading of 158 degrees at the heat inlet to the drum and 122 degrees at the exhaust vent with good airflow out. That being said I'm not too sure where to go from here. Don't really want to buy a new dryer just yet!

I tried a digital thermometer with probe in the drum...it stops reading at about 157-159. I used duct tape on the safety switch and watch the fame at the same time. Good flame, hitting the spoon you mentioned, not near the high limit thermostat above it. Used the infrared again and it got to 161 and the flame shut off. It came on again in a few minutes but shuts off at 161 degrees. Does this mean that thermostat is bad? thanks again!

If you've jumped the hi limit and it still shut off at low temps all that's left is the operating thermo and you've changed it. I wouldn't expect the timer to be so consistent so I think that can be ruled out.
At this point I'd have to start looking for which component is opening. Each thermostat should have 120v to groundbon both sides if closed. If you don't mind working on live devices I would do it by checking for potential across the thermostat. When a thermostat is closed and allowing voltage to pass there should be 0 potential between the two terminals. when it opens that should go to line voltage of about 120 volts. So there's a potential difference between the two due to the open circuit inside the thermostat.
That's all well and good for me to say because I have become comfortable working on these things live but there is of course significant risk doing that when there is a spinning drum inches from your hand and lots of live terminals.
If you want to throw another 10 to $15 at it you could try changing the high-limit and see if that restores higher temperatures but you can also test for that as I just described and access to that one at least it is pretty easy. The operating thermostat located on the blower housing is less accessible.

That level of risk is up to you if you want to proceed with testing on alive dryer. I was very nervous for many months while I learned to work on live machines and 20 years later I still avoid doing it unless I absolutely have to.

I'm on a surprise all day road trip right now so I don't have easy access to look at a wiring diagram on a bigger screen but as far as I know the only thing that controls your temperature is the operating thermostat and the high limit thermostat. I should be back home tomorrow evening so I'll look again then.

I replaced the high-limit thermostat yesterday with Genuine Whirlpool Part:
WPW10423382. Was fairly hard to reach with big forearms, but got it mounted and did a load of laundry. The dryer worked just as it used to for years. Clothes dried in 50-60 minutes on Normal/Permanent Press (Med/High).
Thank you for your assistance. Much appreciative!