Our kitchen faucet normally has excellent water pressure. However, of late, it has dwindled to 50% of normal pressure. If our water has been off for any reason, when it comes back on there are pieces of junk that are stirred up and will have to be cleaned out of the aerator and screen. I have checked this butth epressure is still low. I have taken off the handles and taken apart the hot and cold water valves but cannot find anything wrong. could it be in the shutoff valve under the sink?

Yes, especially if you have turned it off & back on lately. If it's old, can have mineral deposits that may need to be cleaned out by turning off the main water and removing the stem. Clean out and even turn on water briefly to 'blow' it out. Place towels around to catch the water. Replace and turn back on main water. Hope it works for you.