This is based on when i had a rs turbo cabbys. Once i hadn't tightened a jubilee clip on the boost hose that joins the intercooler to the crossover pipe and it popped of which caused it to stall couldn't start, it popped on a right cu#t of a hill on a major road too, was my first taste of proper rs turbo ownership - fix one problem and another one pops up wanting some more money, and when it was running perfect with absolutely no probs and a joy to own someone stole it! back on topic though, i think the air leak could cause a problem and should be sorted, also maybe check your earth wiring too it that doesn't solve it. That cause mine to run fine one minute then cut out the next, i traced that to a earth wire by the radiator!