Why would a furnace overheat with a standard density filter?
It could be that the air return system is blocked or insufficient, and cannot provide the proper air flow.  Make sure your air return vents are not blocked by furniture or rugs, and possibly clean the return air system.

You will also want to make sure the heat vents are open and clear of obstructions, if the warm air has nowhere to go it will not flow properly.

You could also check the blower, as it might be getting old and not have the strength to move the correct amount of air.  The blower can also become dirty and be blocked with dust and debris, which will cause a decrease in performance.

The final thing to check would be the high limit switch itself, as it could be on it's last leg and need to be replaced.  

Remember to change your filter monthly to prevent it from clogging.