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Most likely your DMD chip has dust or hair on it. You can try blowing it out with compressed air but the best thing is to tear down and clean the dmd chip itself. Very carefully. Here is a video of a similar unit. SO far all the DMD projectors I have worked on have had a very similar set up. Take pictures as you go. MItsubishi XD206u DLP Projector Lens Assembly Cleaning
Are they sharp and defined, or 'blob'-like? If they are sharp, that would indicate a faulty DLP block (not worth repairing, unless under-warranty.) If they're more 'blob'-like, it's probably just dust in the optical block. That can usually be remedied by removing the lamp, and blowing-out the unit with an air compressor. Canned air won't do the job, as there's not enough pressure.
I am assuming that the "bubbles" are most prominant when the picture is dark. If that is the case, it is most commonly referred to as dust "blobs". Dust particles get on the lens assembly inside the projector and are just out of focus so they appear as blobs or bubbles. If you can carefully remove the cover and gently blow out the lens / color wheel area with compressed air it should reduce the "bubble" effect. I would not recomend any further dissasembly as it is complex and time consuming and unless your very careful, could end in disaster. Don't forget to clean the air filters either.
Did this start occurring after you replaced the lamp? The reason I ask is I had the same issue after changing the lamp and it was caused because when I took out the old lamp I noticed dust in the lamp housing which I attempted to blow away. This caused the problem because some dust got further into the projector, specifically around the color wheel. I sorted it out by taking the projector apart following the instructions here: http://tantht.blogspot.com/2009/02/how-to-remove-dust-blobs-from-optoma.html (note my HD70 was out of warranty, if yours is not, I would recommend contacting Optoma to get it fixed). Follow the above instructions to step 14 and then locate the color wheel which is at the front of the projector behind a dark gray, plastic type square shaped cover. You have to be very careful here, but remove the color wheel (there is one screw holding it in place) - note it
the yellow blob is from a fried polarizer on the blue lcd panel due to excessive dust and heat. you'll have to replace it, but hitachi doesn't sell individual optical engine parts, so you may have to replace the whole optical engine.
blow out any dust with canned air. chances are there is an object blocking the image. or there is a crack or burn spot on the 2nd fly-eye lens (diffuser). the first one is right next to the lamp (if you remove the lamp, you'll see it). the second is right on the other side of it.
I had an issue with "dust blobs" on my HD70 and was unable to locate instructions or a tutorial on how to remove them. I figured maybe this could help you if the hair is located within the same location as the dust. Please check out my blog at http://tantht.blogspot.com/ for instructions on how to remove dust blobs from a HD70 as this process worked perfect for me and I no longer have these annoying spots when watching movies!!