[Solved] Check Engine light flashing, engine stalling.

The problem has been sorted out, but I leave the topic here for further reference. Solution to the problem at the bottom of this box↓

The problem started when driving downhill at ±85km/h. I slightly let go of the gas pedal to reduce the speed when the CEL flashed for 1/10 second. It appeared that the engine stalled while the light was lit, as the car gave a substantial whack. Four days passed without it repeating, but when driving to work on the fifth (25 kilometers) it happened again four times; 10 times in total that day. Next day it was 32 times to work. Lost count driving home, but at least as many. Seventh day I counted 97 to work.
Pushed it to the limit the eighth day and when passing ±6000 rpm I heard a strange ticking noice, like a crackling relay. Since that it's been flashing and stalling like there is no tomorrow.

The problem now occurs most often when the gas pedal is held in the same position for over 1 second regardless of position. Sometimes it goes in an even rythm, like once every second or so, sometimes irregular with long and short flashes, or completely lit. It seems to continue as long at the gas pedal is not moved.

The (1)CEL can flash, the (2)engine can stall(car jerks/whacks) every time and for as long as the light is lit. The light will also (3)stay lit for longer periods, sometimes accompanied with the (4)engine seemingly loosing a little power, but otherwise running like normal. If I give a little more gas when it flashes and jerks repeatedly it will switch to 3 and 4 symptom. If the rpm increases considerably when 3 symptom is present (running downhill with gas pedal held stationary) it starts running unevenly.

It never happens when idling, and does not occur if I ''bounce'' the gas pedal ever so slightly while driving. The engine has never stopped completely.

Parts tried from a known good engine:
Cam angle sensor.
MAP-sensor.
Both coils.

I have also tried disctonecting the EGR and adding an extra ground cable between the engine and body with no result.

Does anyone have any idea? Do the car store fault codes or will they only show up when CEL is lit (my neighbour has bought but not yet received a diagnosis system)?

It's a 1999 model with G13BB engine.

Solution: My engine has a little, red air filter (picture in comment further down) screwed into the intake manifold, far rear end towards the fuel rail and EGR. The MAP sensor is connected to this with a hose. I changed the filter and the car has been acting like normal for +150km (and counting) now. Hitting it with compressed air made it spew beige oil. All the oil could possibly have obstructed the air flow leading to faulty MAP detecting.

Sounds like a crankshaft sensor on the way out maybe!

Forgot to add that it's a 1999 G13BB.
There is no crankshaft sensor on the G13BB(?).

I discovered the (paper clip-) diagnosing procedure (Service manual 8G-12) and got code 21, ''ECM / Immobilizer Control Module code''.
Any experiences here?

There is at least one sensor: crankshaft or camshaft.
Although the immobiliser code seems to be a surprise - I'd try locating the immobiliser, disconnecting it and bridging its connector, but I'd need to check wiring diagram for that. I worked around an immobiliser problem this way in a Toyota Yaris a few years ago.
Then, if this solves the problem it's up to you: get rid of it forever or replace it (and recode the keys).

But again - that is one wild guess

I'm suspecting the ECM, specifically throttle or ignition related. I don't understand how the immobiliser can be related to gas pedal position, movement and lack thereof.