One shower runs out of water in 7min.. Other stays hot forever!
So this is happening in separate bathrooms that is supplied by one water heater?  You'd be surprised how quick you can go through 38 gallons of hot water especially with a high flow shower head.  Newer shower heads are usually rated at 2.5 GPM, while older ones were much higher.  Also check the timer on the heater.  One other question, is the hot and cold water tied into one shower valve? Or is it two separate valves for hot and cold?  If it's only one valve that ties the hot and cold together, that may be the problem. They use rubber parts that get swollen and wear over time and the valve may be the problem. That would explain lukewarm showers due to the 50/50 cold and hot mix.