I'd put money on a bit of dirt in one of your hydraulic compensators ('lifters'). These have tiny oil holes that can plug up temporarily, which prevents them from pumping up fully until the oil is warmer (more viscous) or the blockage flushes itself out. Try a bottle of your favorite oil additive/cleaner to start, that may do the trick. If not, use a mechanic's stethescope (or the old screwdriver-on-valvecover) to locate the ticking rocker and replace the lifter. There's lots of info here on how to do this job...hope this helps.
