Sudden loss of performance during driving.

Hi all, I have the following problem, last week, during the drive, the car went out suddenly. It turned out to be difficult to start again. After a few minutes and with some gas, it jumped again. But it has the problem that it can only be started hesitantly. Since then, however, there is the problem that the car loses massive power from the 2nd gear. When I come in the 2nd gear to about 3000 revolution, it suddenly calms down to 1500. hl, the car suddenly goes out again, which it didn’t do before. In idle mode, the car can be brought up to the top. I was in the workshop with the car, they said they could not find anything on the petrol line, but they said that it is very likely the cylinder head that is broken. Now I wanted to ask if it can really be on the cylinder head or maybe on something else. Data about the car: Mazda 323 V BA 1.5 16V (BJ 1996) Thank you