85 buick iron duke 4 cylinder running problems....

my son's buick with about 150,000 easy miles has been running poorly....he's changed all the normal stuff plugs, wires,dist cap...fuel filter....cars starts and runs great for short time...maybe a half hour or so....and then it begins to act up...very difficult to give it any gas before it stalls out....always first back up but does not run right....egr valve? tps sensor? ignition module? what do you think is probably the problem? would appreciate any and all suggestions....please copy my email address without the spam in your reply. regards

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