It shouldn't be the valves. If the valves weren't touched when the head was machined, but they were removed and not replaced in the same place and rotation they were in when removed they may be leaking... this isn't something that goes away on it's own. If you had a valve job done they should seat perfectly. Do a compression test if you really think the valves aren't sealing. Did you verify all the valve shim clearances?
As for timing, are you checking the ignition timing or the physical timing of the engine? The timing belt may be off by one tooth on one of the cams or the CPS/distributor drive gear.
Also make sure you're getting a strong spark to each cylinder and make sure all of the engine/ECU/battery grounds are clean and secure.
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