Locking wheels on 2003 Forester

I have a 2003 Forester 5 speed with over 200,000 miles. Recently the wheels have locked intermittently, after stopping, while on flat ground. My husband (who is an experienced mechanic) says it is the calipers but my mechanic says no - he thinks it is the hill holder. The mechanic has not been able to duplicate the problem. I had to be towed the other day and drive a lot so would like to get this resolved (pressing the break to release the other day did not work). Anyone else with this problem - how did you resolve?
Thanks!

Do all the wheels lock up or just one?

The hill-holder will lock one front and one rear wheel (I believe left front and right rear) when applied, so if the trouble is with those wheels then we can assume the hill-holder valve is sticking.

RemcoW and asemaster - thank you for responding! I am not sure - when the tow truck came, he was able to pull it onto the flat bed but against it’s will - it looked like at least the one rear wheel I could see was moving (the left rear was the one I could see - the front ones were not on the ground). I think I am going with the hill holder option. Thanks again!

Yeah, the hill holder sounds like a reasonable culprit. That’s why I asked about all wheels or just one locking up.

The hill holder is a common problem. I disconnected mine the first time it locked-up.