[Capt_Mac]

If you were at all working on the motor, you may have a loose ground wire. I had a problem similiar to yours and it turned out to be a loose bolt holding several ground wires to the block located above #1 or # 2 piston(can't remember exactly). You may want to also check the compression in all cylinders, that could cause serious lose in rpm's or a lower idle such as 400 rpms.