If you get a new cap, maybe get the 1.1bar cap from a 626.

Gurgling has reduced a lot with the extra 0.2 bar of pressure.

 

But potentially more pressure on the coolant seals of a engine with >100k.

 

Ah sad to say but I was seeing a similar issue and it was my coolant seals, requiring a complete rebuild. My coolant was producing consistent bubbles even when cold up through the coolant housing. It turns out that the casting was fragile and had an imperfection, allowing the coolant seal to plug it when it expanded when warm. Try all of the cheap alternatives and tests first but know a rebuild is possible. Goodluck

 

Once you've torn down an original engine and seen the condition of the coolant o-rings, it's easier to appreciate how fragile they become after 20+ years of use.

One reason I recommend running the lowest coolant pressure you can (0.9 bar for most folks, 0 for Evans users) and changing 50/50 coolant every 18 months at the longest interval. Years back my dad had a coolant seal wall failure after habitually topping off his coolant with bottled water. Found broken iron coolant seal walls and massive pitting in his iron and rotor housing coolant passages.

frankly, an original engine still truckin' along 22 years later is pretty damn impressive and speaks to good maintenance and care by the owner