I had a similar problem with a Galco King Tuk leather/kydex hybrid IWB holster (similar to a Crossbreed Supertuck). The gun seemed like it fit snuggly into the holster, but after an hour or two of normal body movement, like you, I would notice that the safety was off. On mine, the leather sweatguard portion of the holster apparently was pushing the safety off (never figured out why it always pushed "off" rather than pushed "on.")
Personally, I finally went with a more conventional OWB leather holster (with sweatguard) and have not had any further problems. It's a nice, heavy duty stiff leather holster, molded to a 1911 frame. The mouth of the holster is still nice and stiff, even after several years of daily use. And, after a few weeks of using that holster, it developed a nice, deep, natural divot from the safety pressing into the leather. I consider that a great "feature!" I've never had a problem with the safety moving while using this holster.
And with it being molded, it has good natural retention. No, it does not have a strap for retention (or which would stop the hammer from falling). But, with good retention on the holster, and with at least 3 safeties (it's a series 80) on the gun, and the rarity of a catastrophic seer failure, I decided I could live without an external strap between the cocked hammer and the firing pin.
Anyway, good luck on finding a fix for your problem!
