Why does our water switch from scalding hot to freezing cold during showers?

First off, we're renting - so we don't have a lot that we can do for this problem as our landlord does not seem to care. We just moved in 2 months ago and have been having the problem since then. No matter the problem, he doesn't fix anything - an example would be duct taping a piece of wood over a hole in a bedroom door. So now that you know what I'm dealing with . . .The faucets are very sensitive to get a warm shower and even if we do get it right - within 5 minutes we get the cold/hot problem. The water heater is not extremely old, but not brand new either - I cannot find the year on it. It crackles and pops so I'm sure it has not ever been drained of sediment and we are in a very high sediment area. But I have been told that if it has not been drained regularly, it won't help to clean it now. The pipes are very old as the house was built in 1928 and barely anything has been upgraded. The water pressure is fine, so I don't suspect pipe buildup, but maybe mixing of valves somewhere?