Dudurin

Hello everyone, In the name of efficiency, here are some facts: 2008 Skoda Fabia Combi 370.000km (231.000mi) 1.6 petrol (BTS) 5 speed manual Fault code: P0420 - Catalyst system efficiency below threshold. On to the issue: Recently started education and required a cheap mode of transportation to pot around in and got my hands on the aforementioned car for dirt cheap. It had a few (minor) issues, which I have rectified, but I simply cannot figure out this damn idle issue. When the engine is warm, the idle will occasionally sit at around 1200-1500rpm, but this isn't a permanent thing. If I approach a red light and stop with a warm engine, while keeping my foot on the brake, the idle will stay at the aforementioned revolutions. If I then release the brake pedal, the idle will drop to normal and stay there. However, if I reapply the brakes or even pump the pedal, it won't affect the idle what so ever. I've recorded a video of this behavior for your viewing pleasure. That being said, this doesn't always resolve the issue. Sometimes, I can l can release the brake pedal and the idle will stay at 1200-1500rpm. When I bought the car, it had an exhaust leak, which I assumed was the culprit of the code. This was not the case. O2 sensors have been scoped and are functioning as they should. I've doused the entire engine compartment and running engine with brake cleaner, but can't find any vacuum leaks. Could an internally defective brake booster be causing this? Any input would be highly appreciated.