94 Toyota corolla 1.6 16 valve losing spark at spark plugs

had in shop 3 times now for losing spark each time they put a distributor in said it's burning them up and on last time they put a used computer in and said to try it each time it last about a month.I have finely started looking at it my self have spark at coil but not at plugs found a bad cap so replaced cap and rotor button that lasted about 6 days then no spark again found bad rotor button check wires and plugs all look new and ohm out fine but did find oil on top of plugs could the be cause the rotor and cap to go bad

scope the secondary.

Do you mean the tip of the rotor arm is getting erroded away by the spark ?

don't have scope and no tip is not getting eroded away first time check cap and there was no ohms from coil pin to center pin for rotor so replaced both rotor and cap then 6 days later no spark at plugs cap ohm out fine so swapped rotor and working so far