New discoveres

The last few days have been really cold here and I noticed that my car hestitates when shifting into all gears. When its warm it will not start to hesitate until it get up to 65mph. I have changed the breather hose because it was cracked and any other visible hose that looked dry rotted. The reason that I took it in to be worked on originally was because the heater only blew cold air now the heater works and this problem has arisen.

Also, where is the MAF sensor located?