The short answer is yes. A coil can run ok when it's cold and fail under load once it heats up. That's not a rare failure pattern at all. As for the CEL light, I think you misunderstand how they work. They don't go on "instantly" in the case of a bad coil. The computer has to complete a "drive cycle" first. It has to see X number of misfires before it sets a code. A drive cycle varies for each particular code, but it's X number of starts, driven at X mph, for X amount of distance, etc.
In the case of a misfire, the computer looks at the crank sensor to see RPMs. It knows when it asked each cylinder to fire and so it expects to see that cylinder's contribution to the RPM. If it sees the crank slow down, it calculates backwards to see when each cylinder fired so it can find out which one isn't pulling it's weight. It also looks at data from the pre and post cat O2 sensors. If it sees a rich mixture that coincides with a calculated misfire, it adds that into the mix. Then, it starts accumulating misfire data as it goes through the drive cycle. If, at the end of the drive cycle, the misfire count exceeds a factory limit for a cylinder, that's when it flips the CEL light.
If you have a scan tool that's capable of reading live Mode $06 data, you can peer into the computer's memory and see how many misfires it has recorded and how far along it is in its drive cycle. (Although, back in '98, computers were far less sophisticated, especially Ford computers). | 
