Years ago, I experienced the same problem. It wouldn't upshift unless I tried to force it by backing off on the gas. Eventually, it got to the point where it simply slid into neutral. Finally, I began to experience the problem where it hesitated going into Reverse. All of this usually happened during the first few miles, before everything "warmed" up.
The problem turned out to be bad seals in the transmission. They dried out, allowing tranny fluid to leak past them. With the loss of hydraulic pressure, the trans wouldn't shift properly. Once fully warmed up, the seals softened enough to work most of the time.
I was told that, regardless of what the manufacturer says, you need to change the trans fluid yearly. Apparently, there are additives to keep the seals moist. Sounds like a simple solution, but it appears to work. I've always tried to change the fluid as regularly as possible and have avoided the problem since.
Of course, replacing the seals requires almost a complete rebuild of the trans.
98 GCL 5.2l