Uhhh... That don't sound right.
The car should not keep going at (as you said 35mph for example), until you hit the brakes. Think about it for a second. That's like having the Cruise control on.
The throttle body is not a drive-by-wire (computer opened and closed) unit. It's hard connected to the gas pedal with a steel wire.
When you take your foot off the gas pedal, the throttle body plate should close, and the TPS should tell the computer that the TB plate is closed, so the computer can switch over to the IAC in order to keep air going into the engine and have it be ready for idle... aka, no surging from going from say, 3000rpm to idle. Ofcourse, there are other factors such as the "dashpot" function in the computer, but I doubt that's what the problem is. The "dashpot" in FI engines is all software.
The Torque converter should unlock below a certain speed. I just don't remember what that speed is. And even if it's supposed to stay locked at 35mph, I doubt it will do any good for coasting since when it's locked, it "connects" the engine directly to the drive train, causing more parasitic loss on the motor, which in turn causes it to "slow down" faster.
I'd say you have a problem. Check to see that the throttle body closes all the way after you take your foot off the accelerator. If it does close, then... I dunno. If it doesn't, check the TB spring, check the accelerator pedal to TB linkage and make sure it's not sticking.
I've never had a Mark VIII (or any other car for that matter) that "coasted" forever until I hit the brakes.
