I have a gas dryer with a very long vent (8 ft vertical + 10 ft horizontal) and (2) 90 degree angles. I need someone to confirm the vent is the issue. Dryer takes several hours to dry. The heating element was bad so I replaced it. I have noticed the flame goes on for 10 seconds at a time. I detached the flexible venting between the dryer and the vent within the house and it stayed on for 5 minutes. At that time I reconnected the venting and the flame went off quickly. I have cleaned out the vent so blockage isn't an issue. If the vent is the issue what an I do?

"I have a gas dryer" How do you replace a "heating element" in a gas dryer? Glow bar perhaps?
If you disconnected the vent and it burned 5 minutes but turned off after reconnecting the vent, I believe you have a vent issue. The length you are running is not excessive. My vent is longer and I have no issues drying. Could be a constricted vent. Did you run a brush all the way through the vent? If you Google your model, it should show you the max vent distance. Adding 90 degree elbows reduced length by 10 ft.

Thank you for your response. I'll Google my model. Yes, glow bar sounds right. I cleaned out the vent from both sides but I can't know for sure it is clear because I don't have a tool that can make the 90 degree turn. I tried a fish tape that worked well, but it got caught on the turn. Do you have a tool you can recommend?

What kind of airflow are you getting at the vent output? I use a fish wire and some strong fishing line. Then I connect the vent brush to the line and pull it back.

You may still have a dirty dryer where lint has built up and is causing a restriction. You cannot see it if it's in certain places, like the blower and the blower wheel. Just 1/16" of dirt in cups of blades on blower wheel will kill 33% of air flow or more. Take it apart and clean every air passage with soap and water, then lubricate all bearings with white lithium grease and change the belt. Then you will at least know that the dryer is okay.