Thanks for all this great advice. I was just so shocked at how hot it is in the room, compared to my kiln in my studio in the city, which NEVER heats up the room like this. This is a bigger kiln though.
The kiln is reaching temp at the expected time - but it's cooling more quickly than my kiln in the city - hence one of the reasons I think it's loosing more heat than it should.
I did put an empty shelf on stilts, as Pugaboo suggests above, just above the top shelf, and had no issues in the following firing. I also avoided putting flat pieces on the shelf just below...worried they would crack. It's always so tempting to fill one more shelf when there's just a bit of space left, and glazed platters are ready to load!
I'm going to check the back hinge as Dick White suggests...and for the next firing, I'll take all the good advice here from Dick and Neil Estrick about better ventilation - I'll leave a window open, at one end of the room, and the screened sliding door open near the kiln, - with a fan pointed out the window to move air out - and hope for the best.
Will keep my phone handy to call 911 if I see any flames! (just kidding - don't think this is a real issue).
Thanks again for all the advice! It's great to be able to come here with an urgent question and get such immediate results!