Hello. We have a Whirlpool WGD9600TA0 gas dryer that will heat sometimes but not all times. I had replaced the gas burner assembly about 2 years ago due to a broken heating element. No problems until a few months ago. That is when we noticed that the dryer would take a couple, if not 3 cycles to dry our clothes.
Fast forward to last week. No heat, no dry. I looked on the forums and figured it was most likely 1 of 3 things: the high limit thermostat, the thermal fuse, or the thermistor. I purchased these items online along with a pair of gas valve coils.
When the parts arrived, I didn’t bother checking them with my multi-meter. I figured “I had them, they didn’t cost much, let’s just replace them.” (I never replaced the valve coils) Lo and behold, HEAT!
2 cycles later, no heat. 
I checked our venting. No kinks and no clogs. At this point, I must mention that our gas dryer does not have an external wall where it can vent. It has to go up through the wall (single-story) into the attic where it meets a dryer vent booster. This particular model has the pressure sensor installed so it knows when to turn on or off. I could feel that the unit was trying to turn, but i had a hunch that it was faulty. I removed the duct tape, and looked inside. It was caked on both sides with lint and would not rotate. I turned off the breaker and went to town cleaning it out. Even with it back on (I blew into the pressure switch), I had to help it start rotating ever so carefully. I don’t think the motor is working 100% (I have a new 167 CFM model arriving soon), but it is blowing. I don’t think it is blowing the rated 150 CFM, but I have no way to measure this nor do I have anything to compare it to. After the booster, the vent takes a 45-degree turn to the roof where it vents out (another 10 - 12 feet for a total of about 30 feet with the booster approximately halfway through the run). Furthermore, I went to our local hardware store, purchased a duct cleaner, and cleaned out the lower portion of the duct-work. I was sweating too much, so I didn’t bother with the upper side (I will get that when I replace the booster).
I then went down and checked my old high limit thermostat, the thermal fuse, and the thermistor; all showed continuity. I then removed the new devices and checked them for continuity; all good. I replaced the backing and tried the unit again, venting through the flex hose into the garage. HEAT! 4 cycles later, no heat.
I put it all back together because my wife was a little surprised at how much space a dryer in pieces can take up in the laundry room… So now, from time to time, it works. It appears that if we wait between cycles (instead of running them back-to-back), it works. Furthermore, if we run a particularly long cycle (>= 40 minutes), it stops heating.
This happens if it is either on a timed cycle or a sensing cycle.
What I do know is that the gas valve is not opening to allow gas to flow. When it is heating, I can hear the gas valve opening, burning for a few seconds and then shutting off (and then repeating). When it is not working, I am not hearing the valve open at all. I also know that the heating element is always on (when it is on a drying cycle, not ALWAYS on) - I have seen the bright orange glow while peering through the vents on the back cover.
I have tried to do my homework, but I am obviously missing something. Are there other parts I am overlooking? Your expertise and advice is greatly appreciated. Thanks!