Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble.
I apologize for any delay in getting you an answer from an appropriate tech. I am a home theater and networking systems integrator and have installed hundreds of high grade hdmi over cat6 repeater devices.
There are several options, and any pro grade system will be able to accommodate 300ft (max), but keep in mind that the best performance will be predicated on having a quality cat6 STP cable with proper terminations.
The best extender for this purpose, in my professional opinion, is a product you won't be able to buy directly yourself (only through an authorized dealer):
http://www.snapav.com/p-1521-kit-320-1cat-sm-irf.aspx This is the base product for 1080p and infrared transmission, and will run about $450.
If you need a higher end model that supports 4k, ethernet, bidirectional communication, etc, you're in the $700 range.
These are pricey, but I've installed many of them and never yet had one fail. They are a true pro-grade product.
For another pro grade option, that you may be able to procure yourself, I'd recommend the Atlona products: http://amzn.to/1RMUrSu (note this is a receiver only, and would require a transmitter unit or matrix assembly on the other end). If you're interested in this type of setup, I can explain more.