Engine issue

[luke1834]

'99 boxster; 125k miles
Driving home on the interstate yesterday, I felt a rythmic deceleration...almost like I was tapping the brake, only it felt like the engine was pulling me back (similar to engine braking to control decent on a long hill). The rythm was about every 1-1.5 seconds. Rythm did not change as RPM channged, up or down. When I pulled off the road and let the car idle, the rpms would vary between 200 and 800...again, with a 1 second "rythm" to the variation. I was less than a mile from the house and made it home. No CEL. No fluid leaks. No noise (beyond the air intake matching the idle rythm). I didnt lose power and didnt feel like the car was struggling to drive. It never stalled. Thoughts on a potential cause? Am I safe to drive to the shop (10 miles; 4 Interstate)?

Thanks