Regarding the uncleating of the main sheet...that would have to be a VERY heavy line to uncleat itself. I believe that you are more likely having troubles with the cleat. Either the teeth of the cleat are worn or one or both of the springs inside the cleat are ineffective.
As for lines, I have been researching the types available as upgrades from stock. Apparently there are primarly two main factors to consider:
Waterproof and light - usually has a newer tech shell, resists water absorption, can be harder to uncleat due to this lightness. Dyneema seems to be typical.
Polypro and Soft - Stock line that is inexpensive, soaks up water so tends to be heavier, easier to uncleat due to the weight. Perhaps you are using too thick of line and it is absorbing too much water?
Popular right now is the Salsa line...very soft and runs very well thru the blocks as long as you don't oversize. Not as expensive as some of the newer and lighter lines.
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