Intermittent shift problesm with 99 E40D

[Sparky3854]

I have a 99 f350 PSD running a super chips programmer set at 80 hp, also done an air box mod, and a 4 in stainless exhaust, truck has just over 83,000 miles, and up till now I have had no problems what so ever, that changed this week, while towing a small 16X7' trailer, and passing another vehicle, my tranny decided it did not want to up shift, I managed to limp it the last 6 miles, when I got to my destination, it would not go into reverse, I tried drive to reverse 3 times, finally it went in fine, on the return trip home, I had no problems what so ever, 4 days later, after towing the trailer about 10 miles again, the same problem started in again. I have a trans temp guage installed, and have NEVER seen a temp higher than 180 since I have owned it, @ 65,000, the oil was changed to amsoil, the torque converter drained, and cooler flushed, and also a new filter installed, I would tend to think from what I have read, and heard, that the E40D is somewhat of a "weak" tranny to begin with, my question is, might there be something that is causing the problem such as a sticky valve, or possibly a bad electrical component? I have checked the oil, it looks as good as the day it was put in. I consider myself pretty sharp on diesel engines, having grew up on a farm, and around them all my life, BUT, having said that, I will admit, my knowledge of automatic transmissions leaves alot to be desired LOL any suggestions?