I had an identical problem with a very similar 433MHz TX/RX pair. Seperate antennas, good range for 9.6KHz square waves; bad performance with data or constant values and so on...
I changed my software to send Manchester frames that were similar to the "RC5" encoding for infrared TV remotes and got improved results. Occaisionally though, the receiver would start reporting random data, and for lack of a better explaination, I chalked it up to urban radio chatter. I'm ordering a second receiver to confirm this.
Also, if you're concerned about synchronization (and who isn't), you can send two Manchester start bits to synch the receiver and maybe do some baud-rate detection for good measure. I had my radio pair re-synch after every 14 bits (another RC5 property).
Anyway, keep us posted on this, darviash. I am very interested in these UHF projects.