Just a thought, but I sometimes lose posts too - but I realised why.
When I go on the 'view posts since last visit' page, I often select a post to view and then reply before going back to see what else is new.
Sometimes, when I am impatient, I click the back button to get the the 'view posts since...' page before I have actually seen my message pop up, and thats when I lose my posts even though I have submitted them.
Its just a thought that if you use the back button a lot like me, you may want to consider! I always try now to remember to actually wait to see my post appear in the thread before using that button!