I recently purchased a 1990 v8 5.0 GT mustang and the engine fluctuates from about 800rpm-2500rpm when in neutral, high idle. We replaced the o2 sensors, the EGR valve, cleaned the air filter, and replaced the air flow sensor. It now doesn't idle as high, fluctuating from around 750rpm-1500 rpm when in neutral. The engine sounds really bad when in neutral, sounding like its gasping for air. It takes about 5 minutes of turning the key over and over to get it to start, and the first 15 minutes of driving the car jerks forward like its not in the right gear when in all gears, being unable to surpass around 20 mph. After driving for about 15 minutes there is no problem. The guy I bought it from rebuilt the engine, and everything is stock. Can someone please diagnose the issue?