After driving the vehicle for 15 minutes the temperature gauge goes above 200 and the oil pressure drops. The motor then misfires when pressing the pedal above 55 mph and the service vehicle soon light goes on. It does not do this when cold.  I used a scan tool and it reads two codes: p0335 and p034. What could be the cause of this?  There is not anything visible that could indicate the malfunction?

The vehicle until warm runs off a different set of parameters for the computer as far as timing and fuel are concerned. Temp is not a problem at 200... and it is normal for the oil pressure to lower once warm. ( as long as you have 10 psi oil pressure for every 1,000 rpm's you are ok.)

The two codes cam/crank sensor are timing related related. So I would suspect something that deals with both sensors,Timing chain maybe?This particular engine has a problem with chewing up timing chains. If you find this to be the problem i recommend using factory GM replacement parts. I have had to re-do several that i tried to use aftermarket parts on. The only other thing that comes to mind , dealing with the drive ability, is the fuel pressure. These vehicles are sensitive in the fuel pressure area, but this should not make the light on the dash come on. You may have more than one issue. The other part of the vehicle that deals with timing is the ECM. If you have access to a scan tool with data stream on it , you can look and see if the ECM is receiving a signal.

I'm curious to know if this was an issue before you had the head gasket replaced and or why you replaced the head gasket ???