Originally posted by: Quixfire
First, there is no such code as a "Low Fuel Pressure", it must have been a lean O2 code which could indicate a lack of fuel.
Secondly, if this mechanic didn't even test the fuel pressure, then he is just guessing at the problem.
Most mechanics when faced with a lean run condition or fuel system code will check the fuel pressure first, then finish the diagnosis if the fault wasn't found. Most likely what happened is he sold you a complete tune-up and when he change you fuel filter it increased the volume of fuel to the engine because the old one was clogged with dirt. Even if this fixed it for a couple of weeks, it doesn't fix the main problem with this condition. Fuel pumps wear out when force to push fuel though a dirty filter, after awhile the pump can no longer maintain the volume necessary to support the engine under load, hence first gear.
I recommend taking to a mechanic who will perform a pressure and volume test on the fuel system. Also let him know what was done before, it would be helpful in diagnosing the car correctly.
Good Luck