Shaking Harshly On Idle & Hesitant Acceleration

hi, i have a 2000 suburban, 5.3 v8 4wd, it has 176,000 miles, the car shakes intermittently when it is on idle, sometimes it shakes real hard & i smell gasoline, & when i go to accelerate it, i press hard on the pedal, it wont do anything, then all of a sudden itll fly into speed, the check engine light will come up some days steady & go away, ive recently changed the spark plug & wires, the gas filter, o2 sensors, & for this problem i just replaced my throttle position sensor bc i read in the haynes manual that doing such would resolve the issue, i am lost bc the problem still persists & the check engine light is off but when i take it to the mechanics, they keep telling me to do a tune up, which i already have, someone please help!!! thanx alot...

Do you have a way to check fuel pressure? Also, check to make sure the fuel pressure regulator is not leaking. With the engine off (after running), unplug the vacuum line to the regulator and see if gas runs out. It should be dry (no gas ever). The fuel pressure regulator is on the driver side near the top of the engine, between cylinders 3 & 5, it's round with one vacuum line. Post back what you find and the fuel pressure. Typically the fuel pressure should be 55-62 psi, engine off, key on, air bled out of pressure gauge. The pressure should not drop more than 5psi after 10 minutes after turning the key off. Hope this helps.