Pull the starter and check for worn contacts. Very common on these starters, and cheap to fix. Pretty easy to tell once you see them if they are too far gone. Had the same problem in my truck around 180,000km. Some days would be fine, other days, wouldn't even turn over.

91 SR5 Xtra Cab, 298000km, 4" lift, 56" rear springs, 33-12.5R15 BFG Mud-Terran T/A KM2.....and an '85 axle still waiting to go in.