OK, I cloned that repository, but I'm running into the same problem as before. Also, I didn't mention that setting frequency with --gekko-compacf-freq <value> does not actually change the frequency. The frequency remains the default one (200). I wouldn't rather buy a fan at the moment. I would like to have it working without a fan, even if that means I should lower the frequency, I have no problem with that.
If you're going to run it without a fan, you'll need to upgrade the heatsink and thermal compound. The stock heatsink isn't good enough for just passive cooling
I typically use a start command like this, it uses the tune algorithm to ramp up the clock rate to the target frequency
./cgminer -o stratum+tcp://solo.ckpool.org:3333 -u YOURBTCADDRESS -p x --gekko-compacf-freq 545 --gekko-start-freq 400 --gekko-mine2 --gekko-tune2 60 --suggest-diff 1000
In terms of core voltage setting you want to have it around 1.45V its very sensitive. 0.025V is the difference between it working reliably or not.
Depending on the silicon you've got you might get away with turning down a fraction but I've only ever found the BM1397 to run reliably at 1.45-1.55V
Edit:
addiitonal, I have vague recollections that the BM1397 doesnt run well at clocks below 300Mhz. Cant recall the exact thread, but I'm sure you'll find it on here somewhere.