My life fitness t9i won't hold it's speed and slows down.

SOURCE: Life Fitness 9100HR Treadmill won't start

Typically a non start up of any electrical Exercise equipment raises alot of red flags If this treadmill will not start Check the internal fuse MAKE SURE ITS NOT PLUGGED IN!!!! UNPLUG IT FROM THE WALL
2 If there is NO Fuse and you just bought it new Return it for an exchange.
3 Check your wall outlet for less then 5 Bucks you can buy a wall outlet tester that tells you if your Outlet is good Or Bad.4 DO NOT Attempt to service this item on your own the way they make treadmills you have to have a qualified service tech come out and fix it because the risk of serious shock or death is emminent.

SOURCE: my treadmill life fitness 9100HR won't start

Have you checked the drive belt @ the motor, if it is OK then check the fuse, if it is ok then the brushes in the motor may be worn out