4l60e trans usually shifts to o/d at 45 mph. Sounds like it is kicking in and out of o/d. At 175k you have received a better service life than most if the trans has never been into and you tow with it. There is a selonoid in the trans that controls the o/d lock up. It could be that.
I'd like to thank Grub, bassman and G_Hipster for the replies. Today I tried something and it didn't seem to change so I am now wondering if it is torque converter or not. When I was running at about 45-50 MPH I again felt like it was surging (very minor though) and once I stopped again, I selected non-overdrive. But when I reached that same speed I still could feel it. So knowing the O/D option was not used, I figured the O/D lock up was out of the equation now. I guess it still could be, but I am thinking everything possible now.
As for the 4L60E transmission, I know all about then very well. I did have to rebuild that transmission because of the Sun Shell Gear stripping. So I replaced it with the "Beast". The beast is an after market replacement for the weak GM sun shell gear and it does fix them from stripping. And at that time I replaced all the steels and clutches and seals throughout the entire transmission. And the steels, clutches and seals were not bad, but since I had the transmission down, I rebuilt everything. I mean the total rebuild kit was a mere just over a $100 dollars, why not. That was many years ago. Actually I've rebuild quite a few of them for other people as well. Not really that hard. In fact any iboater on these forums could rebuilt a transmission easily.
I have a few new parts for the ignition system to replace and once I get everything finished,we will see if that skipping surging effect is still there. It is so subtle, my wife can't feel the vehicle doing anything. But usually the driver can. JMHO.