Over the weekend, I drove my MIL's PT Cruiser and noticed the engine was having issues with the A/C engaged. When idling, every ~10-20 seconds the engine would shudder, the tachometer would jump around, and the car felt like it was going to stall. It never actually did stall out. The car never had a particularly smooth idle but it was never this bad. The problem seems to go away with the A/C turned off. I've done the usual stuff -- new air filter, new plugs & wires, ran a bottle of Techron through the tank. I wasn't sure what the next step should be.