sounds like solenoid contacts.It gets worse and finaly when your on flat ground and alone it goes completly out.I think 4crawler sales them on his site.Whatever you do dont replace your original starter with autozone,pepboys,oriely etc...rebuilt in china, mexico lifetime warranty p.o.s. Fix the one you have if you want it to last.

oh what a feeling:
#1 86 xtra cab 22R,EB 261 cam ,Hooker header,2" ex with cherry bomb, Aussie powered
#2 86 sr5 4x4 efi parts truck
#3 87 2wd parts truck
84 supra yard art