I see in another thread you seem to be thinking your battery is bad...I bet this is your problem if so...
I had one going bad over the winter...I first noticed the headlights surging when I used the turn signals, and dimming momentariyl when I stepped on the brake or turned on defrost, etc...then sometimes when I put it in gear it would stumble and almost die...
I was a bit aggravated because I just put an alternator on it a year ago, and thought it was going out. I checked it with a good tester, which actually indicated a bad diode in the alternator, which was what I expected based on what I was seeing...but being methodical, I tested the battery too, and turns out it was barely alive, even though still starting the car well enough...but after a 15 second load test it couldn't even engage the starter solenoid....
New battery, and all is fine now.