dw788 motor is bogging down

Is that happening while you are cutting or it bogs down as soon as you increase the blade tension?
Increasing the tension should not cause the motor to bog down.
The speed control on the Dewalt 788 is known to be problematic, sawdust can creep in it and cause it to fail, maybe that's the problem with yours, try blowing some compressed air in there and see if it changes anything.

Try scroll saw .com site,there are many members there who own that model of scroll saw ,maybe they'll know what's going on with yours:
http://forum.scrollsawer.com/

welcome to the forum. im thinking a video of exactly what youre experiencing may be helpful. my old 788 would have a change of sound with different blade tensions, but never had a problem with speed slowing down.
im thinking a video of what your experiencing may help.

only thingmi could think of causing what im picturing is a bearing problem, but id like to see and hear a video.

Here's a video of what is going on! I would appreciate all thoughts and considerations!

Does it bog down without the blade? I'm thinking maybe the bushings on the (upper or lower)blade holder are sticking?
Try to duplicate the problem without the blade.
I would try to contact Rick(if possible) and ask him what he thinks causing the problem:
http://www.scrollsaws.com/

huh! as ken said, check it without the blade.does it operate allright when cutting?
you have something binding somewhere.
rick would be the man to contact.
another forum to check is

http://www.scrollsawvillage.com/forum

Also check the connector going to the speed control under the black cover. I've seen an instance where someone had a similar problem and the connector/wires was loose.