need help

i have a 95 pathy SE and i've got an engine problem that's bugging the heck outta me. It idles beautifully, but when you step on the brake and shift into either drive or reverse, the thing runs really rough. Also, when you're cruising along at a constant speed, and you've got the rpm's constant (around 1700 rpm) just to keep a constant speed, there's a major vibration that goes away when i either increase the rpm's slightly or decrease them. I've replaced the BPT valve, fuel filter, spark plugs and wires, rotor and cap, and still nothing seems to make a difference. Could a broken gasket on the EGR valve mount be causing this problem?