Sentence:  And as the Musician neyther streyneth the string of his instrument to hye, for feare of breaking, nor lette[t]h it to low for feare of <WSD>distuning</WSD>. So god ... will keepe a meane neyther suffering vs to be carelesselye secure, nor driuing vs for want of comforte to despayre. 
Meaning of the word marked with <WSD> tags:  To put (something) out of tune.