2000 Olds Alero

Can't figure this one out. 2000 Olds Alero with 3.4L v-6 & auto trans. Has
about 134K on it.

The car usually starts fine, sometimes it takes a few attempts.
It idles perfectly and accelerates well. The problem is when cruising at
light throttle, the engine starts surging. You can feel it and also the
tachometer shows it. Also, the transmission seems to start slipping almost
like its getting ready to downshift. If I let off the gas to decelerate, it
seems to smooth out. Likewise, if I accelerate it seems to work fine. The
speedometer goes wacky too in the same manner. At light throttle it swings
erratically from 0 to 60 or so but works fine when accelerating or
decelerating.
No check engine lights come on and the bulb is working. I connected an
Actron scanner to it and it displayed PASS, no codes read.
I think the engine & transmission are mechanically sound but something
electrical is messing it up.
I have heard several horror stories about this

Help would be greatly appreciated

Dale