Shower temperature swings: cartridge or supply pressure problem?

One of the rental bathrooms has a shower that shifts from hot to cool and back again during use. The change isn’t constant, and the tenant hasn’t reported a complete loss of hot water. Before replacing the whole shower valve, I’m trying to separate a worn cartridge from a pressure-balancing issue or a problem elsewhere in the supply.

A few details I’m checking:

- Does the temperature change when a toilet is flushed or another faucet, shower, or appliance starts running?
- Is the shower flow weak, does the handle feel stiff or difficult to fine-tune, or is there any dripping after the valve is shut off?
- Is there a brand name or cartridge marking on the trim or valve body?

If the temperature fluctuates even with no other fixture operating, would you treat that as a likely cartridge or pressure-balancing spool problem? If it only happens when another fixture draws water, is that enough to investigate the building’s pressure, mixing, or hot-water supply before ordering a replacement cartridge?