Does it have points or an electronic replacement? If points, check point gap, and make sure there is distributor grease packed against the block, on the leading edge so the grease doesn't get thrown around and foul the points. Was the timing adjusted to 28' originally? How much did it change when you adjusted the valves? Do you have aluminum or chromoly pushrods? Did you happen to notice any glitter (fine aluminum pieces) on the drain plate when you changed the oil?
What camshaft, rockers, heads and exhaust does it have? Compression ratio?
PS- Oh yeah; what Lane said too.