1989 Olds Eighty-Eight AC problems

Thanks in advance for your help.

For 4 years I have owned this car and it has always had this problem. It is a 1989 Olds Delta 88 Royale Brougham, 4-door sedan, 3.8L, with 65k miles.

The AC in the car intermittently cuts in and out. The air will always blow, but it randomly turns from being cold to very warm. Typically it will blow cold for 10-20 minutes, then cut off and blow warm (outside temperature) for 10-20 minutes, then it will start working again. The hotter it is outside, the less time it will blow cold air. The car was bought brand new by an older woman, and only driven 50k miles in the first 10 years of it's life, then I bought the car from her and have only packed an addition 15k miles since, so it's up to 65k miles now, but has had this problem since I've owned it.

Any information would be much appreciated! Thanks!