I've got a 97 powerstroke that I commute in 8 miles or so each way. Several times in the last 3 weeks or so I've been doing 40 or so and the truck is in 4th with the converter locked up or in third gear and I'll open it up a little, but not floor it to try and pass someone and the tranny will downshift into 2nd and the tach jumps to 3400 rpm. The tranny won't shift out of 2nd gear until I let off the gas and wait 2-3 seconds. I've also noticed that sometimes when I give it gas the tranny will shift into the next gear and drop the rpms to 1500 or so when it should have held it in that gear. It's done that since I've owned it. I've got 155k on the tranny as it was replaced by ford at 15K according to the previous owner. I've towed with the truck and haven't had any issues towing. I've changed the filter and fluid in it and I'm running synthetic fluid. Maybe have 3k miles on the fluid. Any ideas?