My 99 Tahoe with 78,000 miles recently had new cylinder heads, plugs, fuel filter and fuel pressure regulator put on. However, now I have a hard time starting it. The problem is occasional and seems to only occur when the Tahoe sits for more than 12 hours. Usually takes 3-10 times to start with a weak crank and rough idle. Performed a fuel pressure test on the system and was pulling 54 p.s.i.. I keep hearing fuel pump, but my fuel pressure test tells me otherwise. Any Clue???