Yesterday, I got a H, start up OK. After 2-4 hours of running, 24V low comes on.

I changed the power supply complete today. Start up OK. After 3 hours of runing in the shop, venous pressure LED goes to the top, arterial pressure LED goes to the top, TMP goes to 520, temperature goes to 42 degree (I don't think it really goes up to 42, but I forgot to actually check the temperature).

Turn it off, turn it on again, 24V low comes on immediately.

Any idea?

Act bd, powerlogic bd or maybe even the blood pump. All 3 are easy enough to swap out from a known good source to see if that's your problem.