Already time for a cooling system rebuild.

Phreakish said
Just to be safe, I'd check the oil and do a compression test.
Any time a cap pops, it makes me think compression into the cooling system. I've seen it at least three times and in all cases the owner spent good money on parts that were already fine (except the radiator that exploded in two cases, but they detonated the replacements too).

I asked my buddy and he said he did a compression test when he owned the car. A few were lower than others, but all were in range.

He suggested doing a Prestone flush before taking anything apart, to make sure it doesn't have decades of gunk built up.