p38 2000 4.0 se......an interesting overheating problem...i need help!!!

Hi all. As stated the car is a p38 v8 4.0 se, year 2000, 69k genuine miles.

Problem started with the temp gauge popping up into the red, and then back down to norm position, all in the space of about 5 seconds. This happened randomly, one or twice over a three month period.

Done a sniff test - Passed
Not loosing any coolant
No gunk on the oil cap or in the header tank.

As a precaution i put k-seal in the system......

This made everything worse. After about 5 miles the top hose pressurised and exploded!!

New top hose fitted
New thermostat fitted
New header tank cap fitted.
Checked breather pipe and return pipe are clear.
Re-filled with 50/50 mix and k-seal.

Now she sits at idle fine. Take her dowe the road she overheats.

New symptoms:

Boiling in header tank
Heaters blowing cold
Hoses are very soft
I'm sure i can hear a fizzing sound when sitting a idle when i'm sitting in the car, but the temp gauge says normal.

My thoughts:

Air in system? - Any hints on bleeding it, please don't say use RAVE as i have a mac and it doesn't work.

Bocked rad? - Bottom hose is i would say half the temperature of the top one. Best way to diagnose this?

Heater matrix blocked? Best way to diagnose this?

Any help would be very much appreciated, as i don't know where to start......

drop all the water back flush everthing out, get a bottleof iron tite put in as per bottle says. if that does it you got foooked block i got 7 months out of mine with irontite but in the end needed a new block. so sorry if this is not what you wanted to hear good luck..

rad feel when hot engine off it should be hot all across core but cooling evenly going down to bottom,heater matrix will blow hot if okay,wouldnt be surprised if block not cracked behind liner opens up when warm closes when cold causes pressuriseation of cooling system ,doesnt go down to oil,as coolant is dispersed by pressure air pockets cause boiling in local area usually heater blowing cold is first sign,you could place bottle on leak off pipe from header tank see if coolant is been displaced before getting hot,,when filling system keep heater on hot and drive up road and recheck

Thanks, what's the best way to get to the radiator?

All fixed. Turned out to be a broken thermostat the first time, and an air lock in the heater matrix causing the second one.

All sorted now, thanks for your help