Here is a cheap fix that I hope works for you. Replace EVERY SINGLE rubber vacuum line. Don't just look at the rubber and say "that looks o.k." No, No, No. Replace ALL the rubber vacuum hose in the engine compartment - look carefully to make sure you don't miss any. The look of that hose is decieving. Make sure you get hose that is the proper size that will make an ABSOLUTELY PERFECT seal on whatever it is connected to. Silicone vacuum hose works great - once it heats up it sticks like glue and you'll be assured there are no vacuum leaks. Also, clean out the fitting on the back of the intake manifold that has a vacuum line running off it to the fuel pump. The inside of that little screw gets clogged and can cause these problems too. Good Luck.