at 30 to 45 mph the tach fluctuates about 200 rpm in a rhythmic up and fall and car shudders slightly like you are driving over patches of rumble strips. 91000 miles very well maintained
If you slightly push on the brake pedal does it go away??
Do you have a CEL on?? (check engine light)
If so what are the codes??
I have one idea. I’m told I’m only a simpleton know-nothing pea-brained diy’er so take that into account. Be that as it may, the rumble strip symptom can be caused by the transmission’s torque converter going into and out of lock mode. That could also result in some rpm changes. Note this may be normal, as the lock mode tends to engage in the 25-35 mph speed range.