Well, the 3-4 shift in the first generations like Jimbolin has is controlled by a shift solenoid which is activated by the computer. If the computer is getting a bad input (say a Vehicle Speed Sensor - VSS that is going bad) it might not accurately command the shift. In addition, a wiring harness intermittent can do the same thing. Finally, the solenoid itself could be going bad, or becoming plugged up. Lastly, you have the prospect of soem Valve Body issues - sticking bores etc. Then finally, I have heard that a transfer case going bad and increasing resistance to movement can produce such a sysmptom, along with bad gas mileage. That's pretty much the list for 1st gens.
The 99 has a slightly longer laundry list, in that you also have an electronic pressure control solenoid, known to need replacement during the life of the vehicle. A host of valve body problems accompany the 5R55E, and it's likely a valve body rebuild and replacement of the EPC may solve the problems for Colet47. See the Rebuild Diary for the 5R55E Valve body in the sticky threads above. I do not discount possible bad inputs to the computer, just am a little quicker to indict the VB in the 5R55E's.
