DONT THROW AWAY YOUR BULB JUST YET!!! IT MAY BE THE COLOR WHEEL!
I have an optoma hd70 and was having the same problems. I also tried the "dust blob" tutorial, but unfortunately it did not work for me (and I ended up getting dust on the lens I couldn't get off myself). I started searching on the net for more answers and found some information regarding the angling of the projector. Apparently, if you tilt the projector with the back raised too high, the ventilation system does not work properly and this increased amount of heat damages the color wheel inside (this causes the wheel not to spin correctly and display a flickering effect...and that's also why it works for a few minutes on start-up but then gets too hot and the wheel skips and creates the flicker).
Luckily, I have a very extended warranty and was fortunate enough to send it in and have the dust removed & color wheel repaired/replaced. Now, I've lowered the stand for the projector to not have it tilting at such a high angle and haven't had any problems yet...but it does take awhile to get effected (had mine for about 1 & 1/2 years)
I know I should have probably just sent it for repairs in the first place...but who wants to miss out on having a projector for two weeks when a tutorial says I could've fixed it in like 20 miutes?!?! Oh well. Hope this helps someone...good luck;)