Hi Guys - '86 Z, 305TPI auto all stock w/ 50k miles.

At highway speeds of 65mph+, I'm getting 500rpm rhythmic power surges under load, like climbing a steady upgrade or passing. Also, around 40mph I get constant 4th to 3rd to 4th gear changes. (not sure if this is the same issue but it has that surging feeling, too). No problem with acceleration from a standstill. It does have a tendency to run rough at start up after it has sat for a week but idles fine after a minute or 2. I run 89 octane consistently. No overheating issues. Plugs and wires changed about 3 years ago and the car maybe gets 1000 miles put on annually.

Any advice on where to start troubleshooting?

Possibly torque converter lock up issues