Car shuddering with heat on but not A/C button.

[XerouShift96S14]

96 240sx SE, 105k miles, anyone else have this issue out of their S-14?I can turn the heat on fine, (gonna need a new blower motor, I can hear this one going 'vrrrrnnnnn', at '4' it gets REALLY loud.), if the A/C button isn't pressed down my car literally shudders, rhythmically at idle, it'll be fine one sec then surge, fine, surge, fine, surge...., anyone got any idea what could be the cause?

[navysnail]

check, clean out and/or replace the idle valve, when you hit the A/C compressor it opens another air valve to give the engine more air, which is stabilizing the car. when the ac is off, it uses the normal idle valve, which, if not functionioning properly will cause those symptoms