A/c set at 77 works all day then goes up to 79 at about 5:00 until about 8:00?

this has happened for the past 3 days. it is set to 77 and works fine all day until about 5:00 pm when it shoots up to 79. will go down to 77 around 8:00 pm. coincidentally, the dryer has been on each time this has happened. will check it tomorrow without the dryer - but could it be related? yes we live in florida but until last year we have never had a problem with the a/c. last year this happened but the temp went even higher and turned out the t-stat had to be replaced. maybe it is faulty?