When you're under-guessing my performance my share % also decreases. There are never any changes in my console to substantiate these radical and frequent drops of performance. My submitted shares and hashrates remain consistent.
Even if your number of submitted shares remain consistent, if everyone else submits a higher percentage of shares for that block than usual, or if more miners come onto that pool because of a profitability spike, your % is going to decrease for that block. Mining involves luck. Both at the pool level and the individual level. Some blocks you get lucky, some blocks you get unlucky.
The hash rate on the site is merely an estimate based on submitted shares. Thus, it makes sense that if your rigs get unlucky and post fewer shares over a period of time, the hash rate estimate is going to trend downward. But, the hash rate in no way affects your payout. You're paid based on submitted shares for found blocks, relative to the number of everyone else's submitted shares. That's it.