No, the rpms shoudn't matter....your coolant cap has a pressure release, so if your coolant pressure gets over the specified psi it should relieve the pressure....it sounds like a fault in the heater core...when you take it to get it fixed again, tell them you want to see the heater core they removed....if its leaking from the core itself then there must be a thin spot in it....talk to the service manager and tell him whats happening and how many times it happend and tell him you want it fixed right this time... If it happens again call ( Channel 8 on your side ) and they will make a fool out of that particular dealership.....and you might be entitled to file the lemon law on it....