Have you checked the main battery cables for corrosion and bad connections?
Like it was mentioned the details of the truck would help a bit.How is the starter relay if your truck has one.Some years don't have a relay in the starting system,adding one helps the load on your ignition switch,which could be the culprit.
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews