Hi all again,
After some more investigation I believe this simply may be a case of the transceiver overheating. I opened the case to inspect the board to see if there were any signs of component damage, and everything looked fine. I powered up the transceiver and I still experienced the original problem, but after a few minutes the TX timings returned to normal and the dB level dropped down to about 23dB.
My radio shack got very warm over the last couple of days (over 30 degrees C) and I while troubleshooting I opened the door to let the room cool down for my own personal comfort. I think that this, combined with the black, heat-absorbing enclosure of the transceiver caused the components on the board to overheat and not function correctly.
Thanks to the nature of the enclosure and the board, the D4D transceiver may lend itself to a cooling system modification for people operating in unusually warm climates.
Thank you, too, to Adam for developing the kit: it was fun to assemble and I'm looking forward to making some QRP contacts!