I need some help figuring out why my Whirlpool LGR7646E gas dryer is overheating. The top surface of it gets scorchingly hot and it shuts off entirely from time to time. This is an older dryer (just shy of 10 years if I had to guess) and did not used to have this issue so I'm a little unsure of what may have changed/broken.

I suspect that this is key so I'll go ahead and mention it right away - when I watch the flame through the eyehole I have noticed that it behaves differently depending on whether or not the dryer is hooked up to the vent tube or is venting directly into the room. With the vent tube detached, the entire flame is drawn back into the chute that leads to the drum. But with the vent tube attached, I can see that a small portion of the flame escapes directly up into the dryer housing. Seems like this would indicate an issue with insufficient airflow but I can't see why that is just suddenly an issue for me.

I also think it's worth noting that this issue seemed to start about the same time that I replaced the motor and blower fan recently but I'm not sure that's true. I wanted to point it out though because at that time I had a lot of various parts all taken apart and re-assembled trying to determine if my motor was bad or not. It is quite possible that I put something back together incorrectly...

I have already tried the following since this issue surfaced:
- replaced the cycling thermostat
- confirmed that the heat does cycle on and off during a load
- inspected and cleaned the vent tubes (no obstructions found)
- confirmed that the blower fan is moving air all the way out of the house
- reduced the length of the vent tubes by a couple of feet by running them on a diagonal rather than a vertical and horizontal with a right angle bend. Total length is now ~12ft with one right angle bend immediately behind the dryer
- replaced the exterior vent/flap contraption

At this point I am guessing that perhaps I either reinstalled some parts incorrectly the last time I had it all apart or that my new motor/blower fan just simply aren't producing as much airflow as the old ones did. That last option seems hard to believe since the old motor was really past it's prime - it was to the point where it couldn't turn the drum once it got warmed up.

Any thoughts? All that I know about dryers I've learned from google so please don't assume that I already checked something "obvious".
Thanks