1995 olds cierra overdrive question

Hi all,
One of my cieras a 1995 with 30k on it seems to drive a bit odd on the highway. when I am doing 60+ mph, in overdrive, if you keep your foot down on
the accel so that it maintains the 65 or so, the engine will seem to go down in rpms and kind of run like it is muffled, if you press down some more, it will seem to go down in rpms and then gradually go up like a curve on a graph. does that make sense? my point is that in humming a long, you cant keep your foot at the exact same spot and have the engine maintain the same speed.
I put overdrive in the subject becasue it does not do this if I am in drive.
Is it safe to drive a 3.1 ltr with overdrive in the D drive gear on the highway/ It has more passing power, but is it ok to do that and not use overdrive?
Any suggestions will be greatly appreciated.
Got this car with only 18k last year from an old lady for little $$ and it is mint but I hope it will last.
Thanks
E
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Hi E, I have the same car, and have noticed a few things about the transmission. It is a 4speed auto, so the overdrive kicks in about 50 mph or so. I think your muffled sound is just the overdrive. I drove mine from California to Ohio at 80+ mph all the way and it ran fine. ( In overdrive.) I cant understand why your rpms go down when you press the gas. ??? If I let up on mine at 80, it will downshift into the lower gear, than smooth out in overdrive. Unless the the car lugs, I would drive it on the freeway in Overdrive.
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