97 Boxster Misfire 1 bank

[m12volt]

Just purchased 97 Boxster , vehicle parked for a long time , starts good , idles and runs good .

The problem appears when accelerate fast , the engine looses power , misfires
When accelerate slow - no problems.
CEL went on and flashing.

Misfire codes detected:

P0300: Random/Multiple Cylinder Misfire Detected
P0301: Cylinder 1 Misfire Detected
P0302: Cylinder 2 Misfire Detected
P0303: Cylinder 3 Misfire Detected

Spark plugs replaced , fuel filter replaced , MAF sensor cleaned

It got a little bit better after cleaning MAF sensor , CEL doesn't come anymore , but the problem still exist

Any ideas?