Help! Series 3 jerks at high rpm and high temp

It was a hot day and I was stuck in traffic jam for about half an hour, I switch on the air conditioner. As the traffic improves I accelerate and reach 35 or 40 miles an hour at 4^th gear, suddenly the temp rises to 194 Fahrenheit or 90 Celsius and the car start to jerks badly especially at high rpm (the engine still on). I switched off the ac, put the gear to neutral and let the car cruise along, step on the accelerator and put to gear again this time at 3^rd gear and it is back to normal, the temperature goes down to 82 Celsius or 176 Fahrenheit without ac on. This happen quite often during hot day.

• My car is Land rover series III year 1975, original 2.25 petrol 3 bearing engine.
• Using Weber 34ich carburetor
• Sanden 805 Ac compressor, ac condenser installed in front of radiator
• I use Mercedes electric fan for the AC condenser (only switched on when AC compressor engage) and use standard cooling fan configuration at the back of the radiator only replaced the original metal fan with plastic.
• Replaced radiator and hose a year ago
• I use standard mechanical fuel pump made by Bearmach
• Temperature is normal most of the time at 82c

Is it head gasket, carburetor, timing, fuel delivery (vapor lock) or cooling fan problems?

That little engine is very much over worked without an air con unit. You're most likely looking at vapor lock because of the high temps (made higher by the air con). I know, there are a lot of 4cyl cars with air con, but they are modern designs with decent horse power.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S (Gone...)

No A/C here, but had similar problems before installing an electric fuel pump. Had connected a fuel pressure gauge T-ed into the fuel hose at the carb, found the pressure dropped to zero PSI under high load and high RPM.

Thanks @ mearstrae. I always suspect it might be vapor lock. The Sanden ac compressor causing too much load for the 2.25 engine.

Thanks @erik88lr. I think that's it! the pressure dropped to zero psi at proximately 90 degrees celsius or it is between that range of temperature when the vapor lock started.