Brain replacing something you're trying to recall with something else.
I also randomly have trouble remembering the name of common things I should not be forgetting so easily.
These curious phenomena are common in ME/CFS, and in fact are mentioned in the
Canadian Consensus Criteria for ME/CFS:
Difficulty retrieving words or saying the wrong word
Forgetting words or names seems to be a random and transient phenomenon: it can occur at any time and with any word or name, but each time it appears to be a random word .
Once for example, during conversation, I could not mentally recall the name of a president of the United States, so I had to say: "What's that president's name, you know, the democrat, a saxophonist, had a dalliance with Monica Lewinsky,..." And someone will say: "Bill Clinton", and then I can carry on with what I was saying. I often temporarily forget a word or name like this, but it is always a new random word every time.
Interestingly enough, as the virus which appeared to trigger my ME/CFS also spread to my friends and family, I noticed most of these people suddenly developed this same word recall problem of transiently being unable to remember common words or names (none of my fellow infectees developed full ME/CFS, but most developed very mild ME/CFS symptoms like increased levels of fatigue, and this word recall symptom).
In fact, when I have conversations with these friends infected with the same virus, we help each other out with fishing for the words that have been temporarily forgotten.
This shows that viruses associated with ME/CFS can produce some of the symptoms of ME/CFS in most people, even if most people don't get the full disease.
Saying a word which is wrong, but is somehow related to the correct word, is also something I started doing. For example I might say "spanner" when I meant to say "pliers" — both words are from the same category of household tools. I found the wrong word I said was always categorically related to the correct word. Though this word replacement phenomenon occurred much less than the word forgetting phenomenon.
Sometimes in ME/CFS, the same categorial mistakes can occur in your actions too. For example, hearing the doorbell, but answering the phone.
The term for having trouble recalling words or names is
anomia.
Saying a word which is wrong, but is related to the correct word, is known as
semantic paraphasia (which is a type of verbal paraphasia).