Manual Reset Limit Switch tripping - Heater Relay Chattering

[SteveD1969]

Sorry in advance for the long post but hoping the details will help. I am a Mechanical Engineer working for a VFD/Motor company but have limited knowledge on Furnaces. My furnace is a Coleman 2880-766. Over the past month or so, my furnace on ocassion has stopped running and I've had to cycle power and it would start again. Today however, the real problems began.
After observing the situation for a number of hours and looking through the wiring schematics, etc... Here is what I have determined thus far.
The furnace will start-up, ignite, etc...
It will run for about 10 minutes and then the Manual Reset Limit Switch will trip. I believe this is also called the Flame Rollout Switch? It is attached to a small rectangular tube coming off the furnace.
When this switch trips though, one of the coils on the heating relay begins to chatter. It is a cube style, mechanical relay. The blower relay is a solid state relay.
I go in and reset the switch and the furnace kicks on for another 10 minutes or so.
After reading multiple forums, many suggested a potential lack of return air.
I opened all my vents, didn't help.
I removed my filter and filter cover, didn't help.
I then placed a fan in front of the furnace to blow cooler air onto the unit with the control panel door removed. Thus far, the furnace has run for almost 40 minutes now without tripping. My only concern with this solution is that I'm "tricking" the Flame Rollout Switch but that the internal temperature of the furnace may still be getting too hot?
I realize this isn't a long term solution but it is supposed to get down to -15F tonight. (I'm in Minnesota).
I'm not looking to get into the guts of my furnace, I'm not insane, but wondering if possibly this is still some sort of airflow issue causing the tripping.
Also, I wouldn't think even if there was a trip that my heater relay should chatter. I do have the capabilities of changing out a mechanical relay.
If it goes beyond that, I'll be calling someone, but still would like some input as to what may possibly be causing this issue.

Thanks in advance.

Steve

[beenthere]

Cracked heat exchanger.

[heaterman]

x2

[wahoo]

I'd call a competent professional as it is not safe to operate this unit in that manner. Don't "tinker" with this unit, get it professionally looked at! We cannot assist in DIY jobs.