05 Volvo 670 cummins engine automatic shift. "SM light turns on dash board"

Hello, I have a 2005 Volvo 670 cummins engine and its auto shift truck. I have a problem that no mechanic can figure out? I took it too 3 different Volvo shops and they just keep saying that the computer is saying its fine. "No problem." When I get back on the road I start to have the problem. When I am driving my truck gets to higher gear and it wont shift and has a hard time shifting. Especially the higher gear. My "SM Light" will turn on the dash. Letting me know that it's a problem. I have already replace the transmission sensors, they said not that so it much be clutch and I replaced that also. I already have spent over $4,000.00 and still nothing fixed. I took it to another mechanic and he thought that it might be transmission going out since I have over 1M miles on the truck. But that's not the problem either. They said maybe because of the hot weather the oil gets thinner and it has a problem shifting, when oil is too thin. I can't figure out what the problem is or what to do? Has anyone had this problem and do you guys know how to fix it? I'm sure I'm not the only one that has had this problem. Let me know please if you know anything. Thanks!