I have a Kenmore Dryer (electric) which is almost 30 years old. Recently it simply stops after a few minutes sometimes 2 sometimes 15. In the past, I have had to tighten hinges on the door. But the door seems ok this time. It's a front loader.

Hi and welcome to FixYa, I am Kelly

There are several things that can cause this:

Fisrt thing you do is make sure you have good airflow. Check the airflow at the back of the dryer with the ducting disconnected. It should push a relaxed hand gently away from the vent connection. If it dies NOT do this you need to find out why.

INSPECT with a flashlight:
1. Duct where the external vent connects in as far as you can see for restrictions / lint (clean it)
2. Remove the lint filter and wash it in vinegar and water then soap and water. Fabric sheet particulate build up on lint filters. It looks perfect but can be totally blocked.
3. Inspect the lint filter recessed area for lint build up. Use a folded wire coat hanger and a vacuum to remove any lint build up.

All air passages clean and still have the problem:
It will be time to open the dryer.

Here are 2 manuals that will help you:
http://applianceassistant.com/pdf/27gasElectricDryers.pdf

https://secured.whirlpool.com/Service/SrvTechAdm.nsf/2cd44500d572193285256a45004fd9d6/3ffad75038c96fe4852571ca006cb80a/$FILE/L-58.pdf

INTERNAL INSPECTION
1. Belt
2. Drum Seals front and rear
3. Drum rollers
4. Blower wheel and internal ducting

On a 30 year old dryer I would replace the drum rollers, front and rear drum seals and the belt before re-assembly.

I am pretty sure you either have clogged ducting (to include the outside the house vent flapper) or a damaged blower wheel.
When the dryer is apart... grasp the motor pulley and with the blower housing open also grasp the and turn the blower wheel. It should turn the motor pulley in your other hand with ease. If the blower wheel / fan rotates on the motor shaft replace it.

Thanks for choosing FixYa,
Kelly

Inspect the door switch to make sure it is seated properly, as it sounds as though it is no longer locked in place and the vibrations of the dryer tumbling is causing it to break contact thus shutting off the dryer. Hope this helped Tim