You need to give us some specifics before anyone can legitimately help you here...
- What are you describing as lag? (what exactly is it doing?)
- When is the computer lagging?
...as far as downloadable software to fix this issue, i don't think any of them are going to be a real "fix" without knowing what is going on. There are several things that could be hogging RAM, there could be hardware failure, etc. We need more info.
Also, have you reset your SMC and PRAM? If not, google it and follow instructions...sometimes this can really help a macbook stuck in a weird rut.