Haven’t had a chance to follow up with this yet but here is what I got from Blackmagic Tech Support:
Concerning the heat issue, do you know if the fan on the underside of the
unit is functioning? It should only get really hot if this fan is not
working.
As far as the output slowing down, what resolution, frame rate and codec is
the clip you are trying to output?
If it is uncompressed HD and you do not have a RAID or fast SSD, it could be
a dropped frame issue which would result in the drive or RAID not having
sufficient r/w speed to play back the video properly.
If it is compressed with something like H.264, this will need to be
transcoded first to something like ProRes before this will play back
properly.