300 TDi head gasket? please help!!!

Hi all. New to this but have a problem already. Got a 300tdi discovery. Starts and runs ok until around 2300rpm. After this it starts running rough and smoking quite alot. Been told it could be head gasket. No usual signs of this. Temp stays fine, no oil in water, no gunk under oil filler cap or raised oil level. Changed fuel filter and air filter but still same. Runs and drives fine until over thay rev range and gets worse with higher revs. Cheers.

Had a look today and seen there is a gap about 1mm between the exhaust manifold and the head with a number of studs missing. Also the exhaust itself is blowing in a few places. Could this be part of the problem?

Certainly wouldn't help, get it sorted, then try.