1989 gmc p/u-intermittent starting problem

I have an '89 1500 w/350 tbi engine.App. 6 months ago,after driving about 10 miles, i shut it off,went back 10 min. later & it cranked over fine but wouldn't fire.After letting it set 10 min.,it started & ran fine & never bothered again until the other day.It did the same thing again & when I got home & tried it, same thing & 10min. later started right up.I used my cheap code reader & got a 43 code(est spark timing circuit low voltage detected or electronic spark control-cicuit problems).I'm not sure what to start with as it's so intermittent & replacing controls can get expensive. I'd appreciate any suggestions:screwy: thank you