I have had the same problem with a hard shift into second for about 100k miles. I'm finally getting around to diagnosing and digging into it. I took it to a shop here in town that specializes in transmissions and they were totally perplexed. They dug through the computers, the filters, the fluid and couldn't figure it out. They thought that my H&S MiniMaxx had it programmed in the tunes so they looked through the tunes and didn't see anything, they looked through the factory shift points and didn't see anything, they cut the filter in half and didn't see anything. They came to the conclusion that it was bleeding pressure and needed a rebuild (for $8k). So I called ATS out of Denver and picked their brain. They said it's most likely a valve body and solenoid. They also mentioned they see these trucks throw solenoids and it's never the same one. I'll be going the valve body route and seeing if that fixes it. If it was a seal failure or pump failure I'm assuming it would've done it by now, since it's been doing it for 100k miles. I also tow a lot with my truck, as I'm a home builder so I'm moving equipment, tools and materials on a daily basis. The shop couldn't believe I had 366k miles on a factory 6R140 but ATS told me that's pretty normal.... I ordered the manual from Suncoast so I'll be reviewing that before doing the valve body and worst case scenario I'll rebuild it myself and buy a scanner and FordIDS and still be money ahead.
