We have quite a few issues. We just moved into our house 6 months ago. The house is only 6 years old. When we first moved in the hot water pressure was awful. We had a plumber come out. He cleared the lines and said it should be fine. Water pressure is greatly improved. However a few months later the hot water pressure on the master bathroom was bad again. Plumber came back and cleaned line. Water pressure is fine. Now the shower water will not get hot. (This has gotten much worse during winter.) Water coming out of shower is roughly 88 degrees.

Also, in most of the sinks the water will get hot (roughly 115 degrees). However, later in the day it won't get warmer tha 90 degrees. There seems to be no rhyme or reason to why it is not at one time and not later. Please help!

and the water heater is tank or tankless?
gas or electric or oil ??

If it is a Tankless, they need more attention than a Tank water heater. There are sensors, electrical components and need to be cleaned of lime and Calcium every 6 months to a year, depending on the mineral content of your water. Good luck.

HOW did he "clear the lines", since that is almost impossible to do for water piping, and usually unnecessary. There are several possibilities, but you need a plumber, and NOT one who "clears the lines", to check it out and find the REAL reason for your symptoms.