did your PML kit come with a relay? or does it simply plug into the stock harness? sounds like you're not getting enough juice to the ballast.

if you don't have a relay, then get one. if you don't know how to set it up, then take it to someone who knows/can; meaning you might want to try an audio-specialty shop who are comfortable and knowledgable in working with relays. should be a simple, cost-effective upgrade, and safer in the long run.

if you do have a relay, then your ballast might be going and time to get a new one. however, it's sounding to me that you don't have a relay.