It is still breaking in. The more you play with it the crazier the thing will act. Don't do more then one adjustment to your skimmer in 20 minutes or so. I would look for a different skimmer when you can afford it. I don't think there is a huge rush. The skimmer is the most important filter for your tank.
That skimmer won't cut it for a 75 gallon tank unless you stock it very lightly. The Remora would be a better choice. BUT it can take several weeks for a skimmer to break in, 3 hours is nothing. Patience!