A recent post by Eva Norlyk Smith explains how heavy credit card use can detrimentally affect your credit score. This is also true even if you pay your balance off in full each month.
It basically boils down to when banks report credit card information to the credit rating agencies. Reporting usually happens once a month, but that is not always the case. However, some report much less often than that.
According to Smith, one thing to remember is that each credit card company has its own day of the month, if it reports monthly, that it may supply the information. Depending on what day your credit card company reports your information and what day you make your payment, they may simply be reporting your large balance and not your payment in full.
A few suggestions from Smith for getting around this problem include signing up for a credit monitoring service, calling your card issuer to find out what day they do their reporting, and asking your issuer to raise your credit limit.