I recently bought a 2005 ltd coupe. It was in pretty rough shape, but ran well. It looks much nicer now, new door panels, interior panels tightened up, but I am experiencing a disturbing throttle lag problem. I do the throttle reset and it works for a while, but will happen again. I have been on two long drives (two to three hrs each) and the problem happend both times. When it happens, I put my foot to the floor and it took forever (several seconds) for the throttle to slowly engage. I know this has to be caused by something wrong as if this was a normal problem no one would drive these cars. What should I look for to slove this problem? I really love this car but it is undrivable under the current conditions.

 

It sounds like a fuel flow problem. Replace fuel filter and check fuel pump output pressure. If the car has been sitting outside for years the fuel tank might have some serious internal rusting problems which can block flow.

 

The throttle body could be going bad, or the foot pedal.

 

If it idles fine but wont run right at high throttle positions, I'd suspect fuel flow to be the problem, or a messed up throttle body. (This assumes you are not still running gas that has sat in the car for months or years).

I'd measure fuel pressure first, both at idle and as someone hit the pedal - fuel pressure should stay pretty damn well stable. My Ranger started doing exactly what you report here back in June.
Idle fuel pressure was 62psi, but speeding the engine up, the pressure would drop to as low as 30psi, and it often would not come back up, either. $1000 for a fuel pump, regulator and filter did the job.

If the fuel pressure is rock-steady and you are SURE you are not burning crappy gas, I'd look at the throttle body, intake housing (look for obstructions), and maybe the pedal sensor - tho you can read the pedal sensor's output with one of the life data OBD readers (Bosch1100, CPR123, etc.).

The engine needs air, fuel, compression and spark. You have compression and spark, but do you have the fuel flow and air flow you need for higher output? I heard once of a car that would not GO, there was a rat's nest in the intake, the engine could get plenty of air for idle, but not enough when you opened the throttle.