Intermittent starting problem, 89 Wagovan

I've been having an intermittent problem with my 89 Wagovan starting. It used to happen very seldom and only when the engine was warm, but has been getting more and more frequent. Originally, I changed the cap and rotor and plugs, but it didn't make a noticeable difference.

It still starts sometimes, and when it starts it runs great. But it doesn't start reliably, even though it always turns over. If you have your foot on the gas at the precise moment it fires, it'll start.

Has anybody else encountered this? Any clues what it might be?