You need a new cap. Check your bottle for cracks. Burp the system of air.
2nded - I had a similar problem on my 1988 about 2 years ago, and it was a crack in the overflow tank.
That said - I had never seen a "closed" system like this before this car, had never even heard of one, and after much fighting and burping and hose checking and fitting and refitting, I replaced the whole mess. IMHO, that system is a piece of kit best left in the scrapheap. Especially in La, you're probably gonna wanna make a change sooner or later. I hear it gets hot there.
I ended up switching from the pressurized ("closed") burp tank to a recovery / overflow tank and "modern" "open" system using the directions on this site:
(I put open and closed in quotes b/c to my mind, they're backwards as commonly used - but apparently thats how the community understands them. The pre-92 system where the overflow tank has no recovery method and has a pressure cap is "closed" - this is what you describe and what scared the bejebus out of me when I saw it boiling in my XJ. The newer system as found on most other cars in existence ever, where the overflow tank is not pressurized (eg cant boil) and serves as a recovery resevoir, is "open".)
Worked like a champ, and he was right on the money for cost/time/effort even though the page is a bit dated. Cost about $250 and took all day Saturday and half a day Sunday for me and my GF, so not exactly Michael Schumacher's pitcrew.