I'm having a problem every so often that my Jetta's acceleration responds very slowly. I have had the car for a year, and it has 118,000 mi. on it. I've put on about 10,000 of those. I have not had this problem much before except a month and a half ago when I had to replace the Mass Airflow Sensor. The engine made a very loud rumbling noise at the time, which has not made since and is not making now. Right now it's just ocassional sluggish acceleration. I can make it accelerate faster by really punching the gas, but that makes the car shift really hard and the tachometer really jump. It has been about 0 degrees F lately (it's January in Minnesota), but I've started it up at -20 F without problems before.
Any suggestions on the sluggish acceleration?