I have a 1994 Caldera Spa connected to a 220V/50A
breaker w/GFCI in the house electrical panel.

With help from this forum I replaced the heater
2 weeks ago, and the spa ran well for two weeks.

However the GFCI has started tripping, and trips
EXACTLY 2 hours after I reset it. All systems
function perfectly during the 2 hour window
between a reset and the next trip.

Any suggestions on what I need to fix?

Try it again.... at the point when the breaker trips, feel it to see if it's really warm. (So, you need to be waiting for it to happen).

If so, then your connections are loose at the gfi. You need to ascertain this first before moving forward with any other diagnostic.

No, the breaker is not warm when the GFCI trips.
[Today it tripped with the heater turned off.]

What is the next most likely culprit for the regular
2 hour GFCI trip?

Since it happens reguarily (every 2 hours) Page-up suggested an over-heating breaker (or over-heating wiring)- something not GFCI related, but FLA related. The breaker will trip for 2 reasons - too many amps (or burnt wires), or ~5 or more milli-amps of current leakage.

I think that's the best place to start- look for overheated wiring. If that's not it- I'd unplug the ozonenator, and it's always possible you hot a bad heater out of the box.

Other things- maybe something is getting wet, a slight drip getting where it shouldn't be- but Pageups course is the first place to look.

Do you have a meter that can test for current leakage (20m/10a)? It would make it a little easier to troubleshoot

Thanks sw... I was going to say the same thing - if it really IS and exact 2 hour thing, then the ozonator is the first thing to unplug and see if it goes away.

The thing with temperature check is very important because if it's not caught early, one can end up with a fried gfci which is no fun (and $$$) to replace.

Many thanks to Pageup and Swine. The recurring
2 hour problem was the ozonator!