We have a home on a well which gives great pressure with only one faucet or shower running.However if someone flushes a toilet or turns on the washing machine we slow to a trickle.The pressure tank shows normal pressure during all this[40 to 25 lbs.].We have owned this house for 3 years and this problem only just recently started happening.
Any suggestions? Thank you,robreuff.

25-40lbs of pressure really isn't good pressure. You can try turning it up to 40 - 55(or around there). Are you sure it has always been running around 25-40psi? Have you done anything to the system that might cause a problem? Are there any filters installed?

Thank you for your reply.We have turned the pressure up to 50 lbs but still have the problem. We do have an inline sand filter which has never been a problem as long as I change it once a month.We did not have this problem when we were here in Jan.
robreuff

Are you sure all of the valves are open(on and around the well tank and filter, that are supposed to be on)? Can you buypass the filter to see if you get full water pressure? Have you touched any of the valves about when the problem started? Sometimes the full turn valves will break on the inside.

This sounds like a piece of solder or gravel worked it's way into the line . Try back flushing the water line . By unhooking the line at filter on house side (run a temp line into a bucket to watch for whatever comes out)(you may not get it out but back to a low spot in system) then running a temp line to a faucet or available connection turn on the water (backwards in the system)let run a few minuts or until something shows up in bucket>

Check the air pressure in the bladder on the holding tank. it should have a schrader valve like a car tire. if low it will make the pump work harder and act as if you have low water available. It should have about 45 lbs . the guage on the water should be around 45 - 65 lbs depinding upon the system you have.
You may also have a filter that is stopped up

Perhaps the filter is installed backwards?

Remove it and then try again without it. Does it make a difference? If the filter is plugged once a month and needs to be changed.

It sounds like the well point filter at the pump is having issues. This much silt and sand can wear down the pump impeller and cause water not to be pumped as fast. The well point acts like a pre-filter, but if the inside filter is plugging fast the well point needs a looking at.

Does the pressure go up and down fast? That is when the faucet is turned on does the water pressure go down and then back up fairly fast. You can see this on the pressure guage.

If so your pressure tank has lost its air and like robertl4 said you need to re-pressurize it.
If you increased the setting on the pressure side of the system you need to adjust the pressure on the tank as well.