Can you explain what is happening when the "misses" field is incrementing on a "show ip nat statistics"? The command refernece explanation is "Number of times the software does a translations table lookup, fails to find an entry, and must try to create one".
I *probably* wouldn't consider this a problem. The "misses" are normal behavior. When the router performs a dynamic translation, it inserts an entry for that translation into the NAT table. Additional packets that match that translation just use the existing entry in the NAT table (and are logged as "hits"). If the entry times out, or a packet needs to be translated that doesn't have an entry in the NAT table, IOS increments the "misses" counter and performs the translation.
Depending on your configuration, it may be possible to reduce the number of misses by increasing the timeout values for the entries in the NAT table.