|First you need to understand the basics of wireless networking.|
Wireless is a convenience. You pay for the convenience with the inherent lack of security, latency issues, and loss of bandwidth.
Ever since i added the second wireless router, my speed connection varies in a sinousoidal for from 650K to 30K. Could this be because the two routers are too close together?
Correct spelling is "sinusoid. How far apart are the routers? Are they operating on the same channel? If they are on the same channel, you need to change them to something like 1 and 11 so they don't overlap and interfere with each other.
My second issue and the most important is that i can't get decent speed between LAN computers.
As I said above, your trade off for the convenience is loss of bandwidth and excessive latency. I bet if you check you'll find wireless clients connecting to the same router have better bandwidth than clients on different routers.
I doubt there's anything wrong with your setup, I suspect the issue is the medium................wireless. If you want consistent connections with excellent bandwidth go wired.