Problems with my hot water heater

1) Air in pipes. No air in cold pipes.

2) starts hot but gets to only warm real fast

3) stays warm for quite a while

4) tried draining silt out of bottom but it only improved performance slightly

5) was having to be reset every month or so (temp was set to low but is now set to medium. temp was set to medium after all this started.)

Is it possible to fix this easily?