98 v70 t5 - Intermittent misfire blips at idle and cruising (no codes)

[sacricide]

Car is at stage 0 (i have an ARD TUNE with catless exhaust, with o2 and air-pump software bypass, its probably in the realm of the GREEN tune with 15psi.)
Car has 90k on engine, with 165 - 170 psi on all CYLINDERS
-New ignition wires, plugs are bosch coppers gapped at .025"
-New distributor cap, rotor
-New PCV
-New Timing belt
-fluid flushed, new sump and seals , only original sensors are CAM, MAF, Crank sensor, IGNITION etc

ISSUE:
After Resetting the system or disconnecting the battery. It will drive nearly as smooth as can be, Then about 300 miles later, assuming this time frame is where the long-term fuel trim is set. It starts to MISFIRE, its a fast BLIP where the RPM drops a few hundred really quick. It may happen maybe once every few minutes, Mostly at a cruising speed. And less frequently if almost ever once in a while at idle.

I have used my stock ECU before, for maybe only 100 miles to see how the car behaved, and it instantly smoother at idle, little to no vibration, my ARD ecu isnt like this, doesnt hop around but you can feel it. and that alone just on the INITIAL STARTUP. Cruising is smooth as well. (its hard to fault the TUNE i dont know how smooth it should be running when its first started and calibrating fuel trims etc)

IM VERY LOST HERE, i have no idea whats causing this. I dont want to start throwing parts at the car as ive spend QUITE alot on it to this point. I will probably slap my stock ECU again for at least a few weeks now to see what happens, if anyone has anything i can monitor or check with a voltmeter, or scantool i have one that does live data.

misc note:
-ive checked the cam sensor wire, and crank sensor wire to see if they touched or were close the the distributor cap, and zip tied the crank sensor to a vac line. It seems to have no effect here.

[erikv11]

Definitely test the MAF, especially if it is really 20 years and however many miles old. Unplug it before startup, then run with it unplugged. Does the problem go away?