1997 Jimmy hesitation problem..

Hello all I am new to this forum so please bear with me..I have a 1997 GMC Jimmy with 4.3 liter (vin W)..The problem started a few months ago.It started with a little rough idle and after the vehicle temp reached about 120 degrees it would clear up.Well now it has gotten worse,It idles rough no matter what the temp is and it is missing and hesitating through the driving speeds of 15-50 mph..I have changed the EGR valve,The mass air flow sensor,changed the fuel filter,replaced plugs,wires,cap and rotor,had the fuel injection cleaned.The problem is still present..With one exception..If you floor it the problem is gone..If you just drive it normal and slow it hesitates and misses.It has thrown no SES light and I have taken it to numerous places for their opinions and they are stumped..Please help me if you can..Thank you for your replies..JimmySLS

Check your fuel pressure.