Since this seems to be fairly common in the digital projector world, I thought I'd share my experiences so it may help others.
I have had my AE900 for almost two years now and have always run it in low lamp mode and never run it less than around two hours at a time. With my prvious PJ, an AE500, that treatment gave me almost 3000 hours of trouble free usage on the lamp before replacement. However, at about 800 hours on the AE900, I noticed my picture would dim and brighten a lot- to the point of distraction in many movies. I originally thought it was the iris adjusting so I turned it off. That didn't help. I then came here and searched around and found many suggestionsand tried them all. I finally came up with something that seems to work although it doesn't seem to be permanent. It works for quite a while and then the procedure has to be repeated.
First, make sure it is the lamp flickering/pulsing. Many people are noticing the dynamic iris on their PJs and assume it is the lamp, but it isn't To make sure it is the bulb/lamp, pause a fairly bright image on the screen and watch it. If the brightening and diming still occurs, it is safe to assume it is not the iris.
Also eliminate that it is panel flickering (LCDs only) by entering the service menu and adjusting the panels, in some cases it appears this was the culprit on some LCD machines. There are threads on this all over the forum for different PJs.
Next, move the lamp to high power with the iris off and turn it to one of the brighter settings, like dynamic, normal etc instead of theater settings. If the pulsing stops, then run the PJ in this mode for 4-6 hours in one sitting. In my case, this eliminates the problem and the lamp can be set back to low power, turned back to theater mode and the iris turned back on although the problem reocurrs every now and then.
One other solution which also worked for me for about 25 hours was reseating the lamp and it seems to have worked for others for longer periods. Remove the lamp (when the PJ is cool of course) and reinstall it. Soe peole wipe down the contacts, etc but I didn't.
If you use high lamp, brighter modes and the iris off and pause the bright image and you still see the pulsing, then most likely the lamp is going to need to be replaced. Either go ahead and replace it, or live with the flicker/pulsing until it dies.
As I said, this is really a combination of ideas from many threads I stumbled on. I hope this works for others like it has me.
Living the HT Dream...now in 4D.