fluctuating temp gauge and cold air

Since a week I notice a problem with my chevy.
- The temp gauge is fluctuating (from 40C-100C). But after adding coolant it is less fluctuating)
- Engine is NOT overheating, and fans work
- Upper and lower radiator hose are hot
- Loosing a bit coolant (just below level)
- Cold air (on freeway a bit warmer)
- Signs of leaked oil/coolant between tranny and engine
- Engine making ticking/tapping noise. Valves ?. But that might be because I haven't replaced the oil filter for 60k.

I suspect the intake manifold, as it has been never replaced on this car.
I'd like to know if i'm right, or it might be something else, like the head gasket or just a simple sensor.

thanks

before I forget. I can buy another venture.
Problems are that the CO2 emissions are to high, and it has an abnormal fuel consumption. So, or buy this one for repair, or repair mine.
Any good tips.

thanks in advance.

(P.S Living in Europe so we need a low CO2 because of the yearly inspections)

Could be intake gasket leak in conjunction with plugged heater core and possibly a failing water pump? First 2 are highly likely.