I am not clear about what you mean when you say the battery only charges every few seconds. Do you mean the charging light cycles on and off, or have you got a meter across it ?
I am not familiar with you laptop, but my wife has a Fujitsu and when the battery is charging the charge light, on the front edge, cycles on and off at a fairly slow rate, and goes to continuous 'On' when the battery is charged.
To put your mind at rest, ASUS specifies the output from your charger as 19V. If you test this with a volt meter with the lead unplugged you should get a slightly higher voltage which should be steady. If you get less than 19V, or the reading flickers on and off, then your charger may be faulty. It is highly unlikely to be your battery. Although one review I looked at said battery life on the K43U is not as good as some other laptops you should get between 2 and 3 hours use out of it. Unfortunately all Li-ion batteries lose efficiency over time, but until your battery life falls well below this, I wouldn't worry about the battery.