This is an old thread, but Artie needs "new" help.
The comeback players that I have taught share a common problem: They have the expectations from their heyday with dramatically reduced level of preparation.
Slow tonguing has one major reason: too much muscle applied. Fast tonguing just "rides" on the air. This situation for comeback players is generally caused by the same reason: Body use sucks and the compensation is to tongue harder to get ignition on the lips. You see, when we are well trained and have a stable base, the lips require very little energy to get the sound started. When our body use and/or breathing suck, we use more pressure on the chops and need dynamite in the form of an explosive tongue to get "decent results".
My suggestion is to take a long, hot shower, and practice immediately after that - leaving no time to cool down, after some long tones and slurs, play 16ths on 2nd line G. Your speed should pick up a lot when you are really relaxed.
Google my Circle of Breath here for more details. All the stuff that you need is already in your head, you are just too stiff to let it out effortlessly.