I've had this truck for 5 years now (7.3 powerstroke) and always been stumped by this issue. Transmissions shops always say the transmission is bad etc. However I started driving it at 210k miles and only in the summer to start of fall it will randomly kick out of lockup if I'm cruising and start flashing the od light (unable to pull codes because it always stops before I can pull over safely and check them) . It also likes to fail to shift into 3rd gear if I suddenly get into traffic and I have to go into second rev it up to 2500 put it in drive and it will go... then it will be fine after 5 minutes of doing that. Long story short the converter clutches died due to unrelated reasons. Drove the truck for 10k more miles without any lockup until I got my hands on a used 131k mile e4od from another 96. Installed it at 250k figured my issues will be solved. That was in May come June 2020 it starts doing it agian every day for 10 minutes it unlocks the converter with no warning signs of slippage or anything. And if I get into traffic or off the interstate it fails to shift into 3rd everytime doing the exact same symptoms as before. I've replaced the mlps and vss and no change. However 1 last thing if I get above 70mph where most of my problems begin and the reason I changed the vss the speedo bounces up and down and it does that year round. So unrelated I assume. It also doesn't seem to matter the actually tranny temp the problem can occur at 150degrees or 190degrees on a hot summer day. It only happens when the weather is 70degrees out minimum. Now the truck has 320k now so 200k on this tranny so it doesn't seem to kill the transmission its just annoying and slightly dangerous if it doesn't go into 3rd when I need it too.
EDIT: ive also changed the pcm believing it to be the cause.

 

That is not how this works.

When the OD light flashes codes are stored in the computer's memory. They will stay in memory for approximately 30 days after the last time the light flashed. You need to read the codes to fix this. The other way to fix this is to replace EVERYTHING. The PCM, the wiring, the transmission, the engine, and every sensor.

 

hmmmm sounds to me like my code reader just won't read them then as I have tried but nothing comes up. Is all i have is forscan and a Bluetooth adapter for my phone. However I agree as this transmission is getting near the end of its life I need to replace everything electrical. Is there anybody who sells a wiring harness for the transmission?

 

Me, vote for properly reading the trouble codes it's giving and then acting upon them rather than just throwing more parts at it. You need a different code reading method. I like the leave the engine running after it set a code to make sure my reader gets what is set as an "active" code versus a temporary or "history" code.

All that said, if I were to blindly attack this without the codes I'd be checking the Throttle Position Sensor (TPS) for dropouts first.