O/D light flashing

I recently bought a '98 Mustang with V6 and the 4R70W automatic transmission. Yesterday the overdrive light started flashing. It also seems to have developed a delayed and harsh shift from 2nd to 3rd gear with medium to high throttle opening.

I hooked up my scan tool (an Actron 9135) while the light is flashing and I don't find any DTC codes. Can anyone tell me how to access the transmission DTC that corresponds to the flashing O/D light?

Thanks.

Will try other scan tool

Thanks for the accumulator spring suggestion. Maybe my other scan tool will show the code.

Anyone know if there is an alternative way of accessing this code? Something like jumpering a connector and reading the blinking lights? I've encountered other cars that had that feature.

Update: Replaced filter and fluid, installed cooler

I replaced the transmission filter and fluid this past weekend. I'm at 90K miles and it looks like the filter and fluid had never been changed*. I also added a B&M transmission cooler to try to prevent heat-related problems.

And the result is: The O/D light is no longer blinking!

But the 3-4 upshift still seems to occur pretty late. I'm going to install a TransGo shift kit soon and while I'm in there I'll replace the 2-3 accumulator spring. The shift kit looks like it came with replacements for the other accumulator springs.

* Because I found the factory-installed dipstick plug rolling around in the transmission pan.