Sounds like you have a restriction in your pipes or the utility has a restriction in the service line or meter to your house. A 3" water main is plenty to serve you and a 100 more houses. This is not the cause. 55 psi is plenty of pressure, most pressure reducing valve come set at 50 psi. What you have is a flow issue. You said your flow was only 2 gpm (gallons per minute), if you have 3/4" water meter at 55 psi you should get around 20 gpm. I would first suspect pex pipe because the fitting( els,tees,etc) inside diameter is so small they constrict the flow of water, but you said you had great flow at one time. So the next thing I would do is ask the utility to take my meter out and see if it could be stopped up with anything( mineral buildup,sand,etc.). They should have some type of hookup with a spigot they can put in the meter setter so they can show you what kind of flow they have at the meter,(this would prove its not their problem). Does the water utility utilize elevated tanks for pressure? If so I don't see how you could have had 100 psi and now only 55 psi. There are so many things it could be,you really haven't given enough info, but i would not buy a booster pump. If anymore question or if you solve please respond. I'd need to know more about the water utility system, your meter size and house plumbing type, any filters, etc. Another thing that might help me out, put a pressure gauge on the closest spigot where the water comes in to the house( should read 55 psi this is the static pressure), then go turn a shower on all the way, then while its running go check the pressure gauge. This will give you a residual pressure. See how much water that shower head is putting out, probably about 2.5 gpm, then I might be able to tell you something from those figures.