Hi guys! Hope you can help me with this strange problem of past few months turning car recently unusable.

After startup there's about two minute sequence when engine is running higher RPM (1OOO). After these two minutes RPM dropped to about 700. I haven't thought about it since engine was running well. This changed, when winter came. When this RPM drop occured when engine was still cold there was evident loss of power when accelerating from halt eventually with occurence of "explosive metal tube sound" somwhere in engine compartment. I just solved this by letting the engine to get warm by idling. I had to push and hold the throttle to get higher RPM because dropped 700RPM would cause near engine stall. From month to month i noticed poorer acceleration. The incredible M50 sound when valves opened was the same but acceleration response was poorer and poorer. Then yesterday on 4th gear at 120 kmh engine lost power completely. Constantly loosing RPM even with throttle pedal pushed. RPM stopped slightly below 5OO and no way of getting them up. When tried to do so by throttle the nearly forgotten explosive sound came again. I shut down the engine and start it again. RPM 75O. Running well. A again went to a road. After mentioned two minutes it happened again. Can someone help, please? I absolutely have no idea what causes engine runnig well for the first two minutes and what then causes the engine power drop and stalls. Thanks.