[LazyPup]

Sliding the air filter out was the key to your answer. When the air filter is in place the return air is restricted which lowers the velocity of air into the plenum. With the velocity reduced, the air in the plenum is getting too hot and tripping out the "High Limit" switch. When you slid the air filter out a bit it allowed air to bypass the filter thus increasing the velocity to its normal level and all is well.

Solution: First replace the air filter. The normal airborn dirt and particulates trapped by air filters are microfine, therefore even a visible layer of the dirt on the filter is indicating the filter is saturated and should be changed.

Next, check all return air duct openings to make sure none are restricted by furnishings or personal items sitting in front of the grilles.

If changing the filter does not resolve the problem you may need to consider having a larger filter installed to allow more cross sectional area of airflow.