Soon after my cam change (from stock to a compxe256), I broke a rocker arm stud. So I tore the engine top-end off and took my heads to not only fix the stud but also port/polish them; but I did stick with the same size valves. The machine shop replaced the springs to more beefier ones, and I replaced the rockers with roller tipped rockers. After putting it all back together, adjusting the valves and timing good enough to get the car going, I have new problems. Doesn't take any longer than 5 minutes for the car to be running before the radiator cap 'opens'. For some reason, its overheating. Not only that, but several times when I was starting it up, it wouldn't start the first try, but instead it would backfire. and finally, vacuum plugs keep turning up missing. I'm not sure if they popped off when a backfire occurs or if they are just coming off for some reason. I still need to finalize the timing and valve adjustment. What do you think the problem(s) is?