'99 Suburban trans slip? clueless

[nm156]

Hello, I'm new here so hopefully I'll explain everything ok. I have a 99 burb 1500 2wd with 130k miles. For about a year now I have had a problem that I think is the transmission. One day it drove just fine and the next I noticed that when I would get up to 35 mph and keep it steady the rpms would jump about 300 rpms and then go back to where it was. This occures usually starting around 35 mph all the way up to around 65 mph. It doesn't do it everytime I drive either. Also at higher speeds it doesn't do it. I changed the fluid and filter hopeing that would solve the problem but no luck. I have taken it to a trans shop and went with the mechanic on a test drive with his snap-on scan tool hooked up and he got nothing. No codes or malfunctions. I have also talked to a number of different dealerships and they said it might be the trans. They said without a trouble code they really couldn't do much. Has anyone else had this problem? Thanks for your help!!

Roak.