mysqld is consuming cpu constantly

Question

my config:

• CPU Intel Xeon (4 kernels) 2.13
• RAM 8G
• CentOS 6.4 kernel 2.6.32-358.2.1.el6.x86_64
• mysql 5.1.67 , databases occupy about 1G

and this is what i have:

1. mysqld is consuming a lot of CPU, as soon as it is started ps -p pidof mysqld -o %cpu,%mem %CPU %MEM 13.8 0.8
2. though there is no requests to it (monitored with mtop)
3. once requests to mysqld started, the %CPU value is rising(and never falls back), even up to 85%

what can be the reason?