I've had this problem for a while now my headlights will flicker when I'm parking etc or changing gears what could this be?

I replaced my alternator a year ago with one of those high performance one's because I was going to install a stereo system but i decided not too in the end.

Could this be my problem?

Check for grounds from the engine to the body, and the battery to the body.

Stick a voltmeter to the cig lighter and see what it reads during that time. If the voltage doesn't drop at all, look at grounds, if it does, look to the alternator itself (with and w/o lights on same conditions)

Look at the power connections, grounds, body to engine grounds (often broken or damaged in engine installs), battery to body grounds (often lost with battery cable replacements)