1989 240 wagon, manual transmission.

Yesterday, I was driving about 60mph, and had the power cut out for about two seconds, like the ignition was turned off. about three to five seconds later, it cut out again, for about two seconds, then on again. This went on for about a minute, and I could maintain speed (there was about as much "on" as "off").

I did not notice any of the dash lights come on. One of them may have, I was paying attention to other things.

For the next ten minutes of highway driving, there was nothing out of the ordinary. I tried to coax/taunt the engine into quitting again, by going full throttle/none. It functioned perfectly.

It doesn't seem like a fuel contamination problem, as when it cut out, the loss of power was complete. When power came back, it was 100% and immediate.

My guess is spark. Where should I start on this? I have a parts car, 1989 240 wagon, automatic transmission. I can swap parts from it for diagnostics.