There are two possible situations:
1) You have some poorly written code that is using more CPU than necessary, which you should identify and correct to reduce that usage.
2) Your level of traffic simply requires more CPU resources than you have. You need more CPU capacity, not RAM.
How to get more CPU capacity depends on who you host with. Some "plans" are just variations on hard drive space, bandwidth and RAM. You need a plan that allocates a higher portion of the physical server's CPU time to your instance.