There's a good chance refrigerant is low if it's getting that cold. Low refrigerant means less capacity for the system to carry heat from the cabin to the condenser. 1998 being a little low is acceptable though so don't be alarmed, refrigerant is slippery stuff, but its a good idea to get some gauges or take it to the dealer and check if your LP side is low or not. Gauges are alot cheaper. Don't take it to "the shop" modern indy AC shops are becoming as bad as the chop shops of the old days(look at prices of HFO-1234yf "Opteon® YF" then look at thermodynamic chart of Opteon and r134a and compare, easy swap and sell, plus an automatic compressor replacement will roll in to your shop in a year or so)). Plus most of those guys don't even know enough to diagnose a problem like yours, all they know is "is she blowin hot? welp shes got a leak!" then they fill with refrigerant and disregard oil or even changing the desiccant. Then they will charge you based off your vocabulary and clothing. </rant>