Starts and runs well. No evident loss of power. Stumbles at idle. 5 speed manual on 1997 Subaru Impreza Outback

Engine returns to idle normally (i.e 750rpm) but after about 15-20 seconds, idle drops radically almost to a stall. Sometimes returns OBD codes P301, P302, P303, and P304 for engine misfire for all 4 cylinders.

Replaced MAF sensor, idle air control valve, fuel filter. No vacuum leaks.

Asked by for the 1997 Subaru Impreza Outback
seek diag amd est from a mech. you could have a primary or secondary ign failure
1 more answer , 3 more comments
check timing belt, but first check all ignition components, ex: wires, cap, rotor, spark plugs ect..
Thanks for the reply!! Ignition components (plugs and wires) are fairly new (less than 5K miles) and coil tested to spec. No distributor on this car. Compression checked out ... all w/i 5% of each other. Any other ideas?
try testing the plug wires just in case, also check for cracked spark plugs, ignition module, throttle body
maybe pluged and spark plug gap
Reasonably sure that the problem is not with ignition or fuel delivery systems. Any idea what mechanical issue(s) might cause this problem?
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