intermittent pressure fault

Hello, I have a 3.9 V reg auto V8 disvovery. When I bought the vehicle the head had been heavily skimmed and had a head saver kit put on it. Drove it for a couple of hundred miles no prob, then it started building pressure. Stripped it and had the heads pressure tested they were fine, put it back together. It runs fine, but starts to build alot of pressure and air gets in the system, it at no point overheats tho. Ive tested it and there are no exhaust gases in the cooling system. As an experiment it will happily run for an hour with the expansion cap off, however sometimes I can start it and the water level will slowly rise and spill over and continue to do so. Why is this intermittent problem occurring? Any ideas.