The leak your neighbour found was a leak at the intake (intake ports) part of the gasket - not the coolant passages. If you have a leak there, it will defiently give you a rough idle and a misfire because it is sucking in air, causing a really really weak mixture at those cylinders. Also if this intake leak isn't fixed, it will get worse and you will be running lean on those cylinder(s) that have the leak, further causing more problems to your engine(overheated intake valve, engine knock). The propane method works really good, and never lies, so if he found a leak with propane, thats your problem.