The gas comes on and off and clothes do not dry

The service manual for your dryer, at the link below, will cover some troubleshooting information.:

http://www.mediafire.com/file/zrlyi1izwly/MDE5500

Assuming your gas is staying on and restarting each time the ignitor glows-the description below will likely lead you to a solution:

This is likely the result of an air flow problem if you are sure the heater is working properly. Ensure you are not running the dryer in a closed room which would starve the dryer of air dramatically reducing the air over the clothes even if the full length of your vent is brand new. Next, remove the vent hose from the rear of the dryer and check for strong air flow at the outlet. If it seems strong enough then run a vent through a window or out into another room and do one load in this fashion. If this resolves your problem the blockage exists in the vent line outside your dryer and disassembling dryer becomes unnecessary.

If not, ensure you have seen and touched every inch of interior dryer piping and hold the lint filter up to the light to be sure you can see through it. Disassemble and clean ANY section of piping you can't get your hand into. The area below the lint filter tends to catch a lot of lint and can only be accessed by removing the housing at the rear on the left side as you face the rear. Ensure the blower wheel turns when the motor turns. (Newer models may have the filter housing at the front)

Having performed all these actions will likely resolve the problem.