Strange overheating

Good evening people,
Got a strange issue going on hopefully someone can help before i start taking engine apart blind.
Last week car started to overheat on the way to work so i pulled over to have a look, water leaking from bottom corner of rad and no coolant in expansion tank, grabbed some radweld and limped car to work, after further inspection the metal fins/water channels on rad had come away from plastic at bottom and was as water tight as a sieve. I had no choice but to get the car home so i filled up best i could with water and managed to get it home after stopping a few times to let it cool and add water.
Put a second hand rad in and great no leaks......BUT,now i drive a few minutes down motorway temp guage stays at normal (3 bars) then it will suddenly go to red start beeping then settle back down to normal and it continues to do this!!! Theres no mayo under oil cap or dip stick, theres no steam coming from exhaust, theres plenty of coolant in expansion tank but it will bubble up when i release cap. When temp goes to red the blowers stop being hot but then return when temp drops to normal.
Ive changed thermostat for a new one but still the same.
Im starting to panic its head gasket or water pump but at the same time im assuming that if it was either of these it would be constantly overheating and id lose all coolant?
Oh and fans working as it should.
Ive left car idling for nearly 2 hours today and temp just stays at 3 bars although fan kicks up and down.
Oh and rad pipes are getting quite solid and hot.
Sorry for the LOOOOONG post but just trying to give as much info as i can.
Any ideas what to try next or am i just kidding myself and im gonna have to start messing with gaskets and timing belts??
Many thanks in advance

Hello Blaconbomber

The intermittent nature of your problem suggests to me that you have air in the coolant system.

If you haven't already done so, try bleeding the cooling system for air.

Be very careful when doing so, ie only when the engine is just warm and off at the time of checking, check that the coolant pipes, especially the top ones are warming up, also the top of the radiator.

Also check that heat is getting to the heater matrix inside the car by turning the heater settings to full hot.

then it will suddenly go to red start beeping then settle back down to normal and it continues to do this!!!

Is the coolant level low when it beeps? (below the sensor level)?

Is there a specific way to bleed a c3?
Im unsure if coolant drops below sensor as it only happens when im driving
Im liking the way your thinking, nice simple things, cos ive got nightmare scenarios going through my head lol

Blaconbomber wrote:Is there a specific way to bleed a c3?
Im unsure if coolant drops below sensor as it only happens when im driving
Im liking the way your thinking, nice simple things, cos ive got nightmare scenarios going through my head lol

Yes, special coolant and a special bleeding, try here for both

Nope tried all ideas, still same thing lol
Ordered a new coolant sensor see if that helps......its so frustrating because car seems to be running "ok"
So now my thinking is (and this could be completely wrong) that if temp sensor isnt reading right then coolant will always be too hot before fan kicks in therefore causing issues (maybe)
Noticed this morning that there are no bars on temp guage for ages then itll go to 1 then ages before 2 then 3 then ill slow down or stop itll go straight to red then come back to 3 when driving again

Blaconbomber wrote:Noticed this morning that there are no bars on temp guage for ages then itll go to 1 then ages before 2 then 3

A thermostat jammed open will make it slow to heat up.

Did find all the bleed points for your engine?

Did you find two temperature sensors? Some engines have 2 and some only 1.

Deffo not tstat, i put a new one in (tested in boiling water first) and the old one worked to in water but did look a bit crappy so changed it just incase.
Only bleed point i found was on heater pipe near bulkhead and ive only found one sensor near to the tstat housing