hi you can use a relay at the starter so what you do is get a four or five pin 12volt relay remove start wire from start solenoid fit a 4mm wire straight from batt pos to terminal 30 then put another wire 4mm onto terminal 87 or 87a doesnt matter from there attach that wire to the solenoid on the starter
then the original wire that was removed from start solenoid is now attached to
terminal 85 and from terminal 86 addd a wire to ground so when you start unit the solenoid pulls in and gives the start solenoid direct battery power this eliminates any voltage loss in the wiring its a pretty simple thing to do and it may help your problem either that or you are getting crap starters and also change ignition barrel to see how you go