Toyota Repair: 92 toyota pickup rough fuel flow trouble code 21, accel ignition coil, intake leaks

Question
I have a 92 4x4 4cyl toyota pickup with roughly 150,000 miles on it, i installed an aftermarket air intake on it using one of those MAF adapters and put one of those breather filters on the resonator since the air box would no longer be used and I thought it would be a good idea to keep that air filtered.  I have also used the lucas oil fuel additive in it as well and have installed an accel ignition coil in it.  Occasionally the check engine light will come on for a little bit and then go off.  During this period there will be rough idling and it seems as if there is no response when pressing the gas pedal, sort of like no gas is reaching the engine. I was wondering if you have any insight on this problem since I can't figure it out.

Answer
The DTC# 21 is an Oxygen sensor code, have the output voltages of the main oxygen sensor checked to see if it's working correctly, if it is there is another problem with the engine causing it to run either too rich or too lean, if this started after the aftermarket air intake installation recheck for any possible intake leaks which I really don't think is the case because of the intermittent nature of the problem but check anyway.
It could be a fuel problem, try replacing the fuel filter and see if it helps.