P0325 diagnostic code. Need help!

We have a 1995 Chevy Blazer. We recently had the exhaust system replaced (all but the catalytic converter, which was fine), plugs, wires, oil change, fuel filter replaced. It turns over a little slow, cranking several times before starting, but once it's warmed up it revs very high, then very low and sometimes stalls out. Highway driving is fine, but when you come to a stop sign or light you notice the idling problems. It was put on a diagnostic tester and code P0325 Knock Sensor 1 Circuit Malfunction came up. Would the knock sensor malfunction cause the vehicle to rev high then low like this? It almost feels like you have your foot on the brake and are giving it gas at the same time. You can feel the blazer jerk like it's going to take off on its own. We are not car repairman and don't do our own work on vehicles, other than oil changes. Is this something costly to repair at a garage? Any suggestions as to what we can do or other things to check before we call the garage would be greatly appreciated. We want to come across somewhat knowledgeable so we aren't taken advantage of at the garage. Thanks!