Throttle Cuts Off When Cruising At Around 50-55mph?

[Och]

Here's a problem that my 94 LS400 recently developed.

When cruising down the highway holding the throttle steady at speeds between 50-60 mph, and RPM at around 1800-2000, sometimes the throttle will cut off, as if I let go of the gas. RPMs go down to idle level, around 650, and the car keeps slowing down. Slightest tap on the throttle and it picks up again. Basically exactly as if you were to completely let go of the gas and then hit the gas again. What can it be? TPS sensor? Throttle cable perhaps?