I had a similar thing on a 306 I fixed a few years back.

I'd flush the heater matrix, with a gurgle and then spurt (suggesting air lock) then it would be all toasty and warm, then a short while later we'd be back to room temps.

The car had a weep from the water pump, and I can't remember from memory now whether it was some sort of radweld/coolant fix stuff that was clogging up the heater matrix, or an airlock due to the leaking pump.

Either way, if you have any sort of leak on the cooling system then, unless it is the matrix itself (damp footwells), you need to concentrate on solving that first.

It could easily be a bad gasket on the water pump, which wouldn't need a new pump, but the lump of the job is in the labour anyway