Excessively worn or mis-aligned timing belt

Replace the timing belt as required.

Worn camshaft lobes

Replace the camshaft or camshafts and SHLAs - valve lifters.

Sticking SHLAs - valve lifters

Replace the SHLAs as required.

Stuck valves

Carbon buildup on the valve stem can cause the valve not to close properly.

Repair or replace as required.