I think I know how to get 100%... do you have to play all the levels twice? If so, there should be a way to know which levels you've already completed.
Bug 1: Level 3-4 i get stuck on the missile lanchers and spin round and round. I have to exit the level.
Bug 2: I upgraded my full strength stat and it worked (when it shouldn't).
***SPOILERS***
Here's where all the gold is - not including ones out in the open or dropped by enemies.
There's 4 at the start, 3 in the credits, 6 in the tutorial, 3 behind barrels in Death valley, 1 behind the tip of the big cactus at the windmill, 2 hidden behind windows in Fort 42, 5 behind bushes in Kiowa Plains, 4 by the rabbit, 1 on the gold town sign, 1 behind the sherrif's door and 3 in the safe.
Space bar