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Strange. Since you say the intake side is very strong, then the return has to match it. What it really sounds like is a leak or crack in the return line, but you would be losing a lot of water. If it were an air leak, you’d lose suction, not return. Same thing if filters were dirty or a valve stuck partially closed. So I guess what I’m saying here is that if your water level is steady, you’ll need to call a pool contractor in.

Sounds like a clog in the system.  I would disassemble and run some kind of snake through to clean the pipes.  If you have ever disassembled pipes that have been in use you know you will find a lot of garbage on the sides of the pipes.  Steel pipes can fill with rust.  Plastic and copper pipes can build up dirt and oils along the sides and in corners.  All this stuff is really nasty and can sometimes be cleaned with a round soft plastic brush at the end of a snake.

Providing your Guage is accurate (no debris where it screws in or just plane in-accurate) the most likely thing is your pump impeller has debris in it.  A wire coat hanger bent to fit into the pump basket area and into the inlet of the impeller can scape out any lodged debris.  Also, if you manual vacuum is on a hose that attaches to your skimmer, it will affect your flow.  It is like adding that much more plumbing to your suction line.  That, in turn, will reduce your operating pressure.  With out looking at it, these are all educated guesses.