coolant dissaperance, engine problem

[Uzelac]

I have a problem with coolant liquid leaking, possibly something more.

It's bigport 3rib 4AGE 16v rebuilded some 3000km ago.
It has Wiseco forged pistons and custom intake (ITB, custom manifold ..),
radiator from some some Honda (big enough), Opel electric fan and thermal fan switch.

Beside this, engine is stock.
When rebuilt, new water pump was installed, all gaskets etc. standard stuff.

Like month ago ago, engine temperature started to raise, coolant overheated and started to evapourate.
It happend quickly, during very slow city drive, outside was like 25 C and rain was pouring like crazy. That's the reason i haven't noticed temperature, really had to focus whats happening in front ,
I stopped, poped the hood and saw water smoking runing out of expansion tank. Only at that moment fan started.
Radiator cap haven't moved, and when smoking stopped, there was still some water (1/4 of tank) in expansion tank. I waited for some time, connected FAN on "allways ON" and went home. Temperature was OK, at the middle of the scale on that 2km ride to home.

Tomorrow i flushed all coolant, all oil and put new one. Also new thermostat, double checked thermal fan switch, relay, cables... everything looked fine.

I drove for some 200-300km, mixed driving, no problems, run great, until...

Few days ago, temperature started to rise again while driving faster. I was quicker this time, shut down the engine before overheating. Scale went on 3/4 of gauge, not overheated, at least according to it. I couldn't figre out if FAN started or not. Few minutes after, i was driving slow, temperature was at middle od scale and i went home, few kilometers more.
Tomorro i haven't checked anything and went for slow short drive.
Again, temeprature started to raise, but this time, slowing down to 2k rpm drive in 3rd gear with minimum throttle couldn't lower it. I stopped,checked coolant level in expansion tantk and saw a unpleasent suprise, there wasn't any coolant.
Luckily, had some water at hand and poured in some 2 liters to fill up radiator completly and expansion tank to full setting.
From that moment, drive was ok.
Day after, that's yesterday, i left it idle some 20 minutes. AFter that, there wasn't anything liquid on ground, couldn't see any water was missing from expansion tank, radiator was full.
I checked oil, it was still at max, looked ok. Haven't smelled it that time.

Today, first drive was some 5km, everything OK. Couldn't see anything is missing, coolant level same, car running great.
This time, when i stoped, i checked oil level and smelled it. I could feel gas or exhaust fumes, something like that, not sure, but it certainly wasn't just oil. Oil level ok.

Second drive was some 20km, easy driving at night, and after some 20minutes i stopped and checked all liquids second after i shut off engine.
Coolant was ok, but oil smelled more on gas, much more than 2 hours ago..After 10 minutes i checked oil level, it was ok, oil looked ok and the smell became weaker.

That's current situation, from tonight.
All this time, car is going ok. I have some problems in 1000-1800rpm area which are constant, but they started after last mapping attempt. These glitches happens every time at that range with easy throttle. If i push it harder, engine passes these rpms without issues.

In all other situation, engine works great and performance are at expected level. Exhaust doesn't smell funny, it looks ok.
I don't see any leaks on engine, but i examined it just briefly, i have to wait for tomorrow or even Friday to look at it thoroughly.

That's about it.
Don't know what to do, can't find were it looses coolant.
Most confusing is the part where it looks like it looses coolant from time to time only, not constantly. Engine runs and sounds great, so it doesn't looks like it it leaks in cylinder or oil.
Oil looks ok, but it smells like exhaust/gas. But, i can't be sure if that is really a problem since my pistons are forged and backslashes (hope i got it alright, space between two parts, inside tolerances) are a bit bigger with them.

I would really appreciate help. Thanks