distributor keep failing 3 in one year going on 4th

so every couple months my timing goes to ****...it takes about 2 weeks slowly getting worse....turning over but no fire and flooding engine ...it always in' s up being distributor....[ 69' 225 slant six 1bbl.....cleaned plugs,new cap...egr vacume is working ....this time though even when had been running for a while fine..,it would drop to low and boggy and eventually push though