My 96 GL idles funny. Upon starting, it will instantly idle at 1500 RPM and then slowly drop down to about 900-1000 RPM. After it idles around 900-1000 RPM for a moment, it will fall down to 500-600 RPM, stumble for a second, and return to 900-1000 RPM. It does this over and over. The car feels as if it is going to die but catches itself and returns to 900-1000 RPM. The car has never stalled out. Temperature does not make any difference. Any ideas on what it could be?