What do you suppose the word is for when symptoms disappear when you see a doctor or worse still the car suddenly works perfectly when you take it in for a service? I’m looking for an expression like Murphy’s Law. (That’s when the sandwich will always fall on the jam side.)
No, Murphy’s law is ‘if anything can go wrong, it will.’ This page which is about the history of Edward’s Air Force Base, where Murphy was stationed, agrees.
I recognise my error in respect of Murphy. However the question remains. What do we call it when we get the washing machine repair man out to fix a leak and the man finds nothing wrong; or our symptoms disappear as soon as we get to see the doctor, or the car engine decides to stop knocking when we get the mechanic to check the car? Sod’s Law? P.s. Am I the only person in the Universe to experience this phenomenon or what?
What do we call it when we get the washing machine repair man out to fix a leak and the man finds nothing wrong;
We call that typical for Sears.
No, you are not the only one to experience these frustrations. If there is not a term or phrase for it, there should be.
How about karma-baiting?
Sorry, I’ve never seen that one given a name–it’s just one of Murphy’s laws.
LOL. And of course, this is a good example…
Regarding the OP:
Maybe this is related to the “skeptic effect” psychics always compain about. You know, how the mental power of someone actually doubting their “ability” is enough to disrupt any demonstration of that “ability”. Maybe we just need to convince our doctors/mechanics/etc to keep an open mind. 
I just chalk it up to the innate contrariness of inanimate objects.
How about this:
Law of Inconsistent Symptoms
The odds of a problem occuring while being examined by a specialist are inversely proportional to the product of the expertise of the specialist and the amount being charged for the examination.
Regarding the Murphy hijack:
My favorite phrasing has always been “Given a task with only one right choice and only one wrong choice, a person will do it the wrong way 75% of the time.” Of course, after working tech support, I came to realize that 75% might be an optimistic estimate.
I’d heard that the version of Murphy’s Law that applied to “When you try to show someone a machine doesn’t work, it does” is called "Willoughby’s Law. I might’ve read that in one of the Murphy’s Law books floating around.
Yes. Yes it is. 
Thank you for your contributions. Maybe we can apply the effect to the awkwardness of not finding more than two rusty vintage oil barrels container fertilizer, when one has 350,000 troops waiting at the desert border just waiting to go in. You know and I know there are probably thousands and thousands of nasty weapons hidden away; but gosh darn it can ya find em when ya need 'em?
My years in tech support had lots of, “I dunno how I fixed it but it’s working now” situations.
We usually logged it as “Came clear while testing.” but occaisionally would log it as “PFM” or “BFM” for (politely) “Pure Freakin’ Magic” & “Black Freakin’ Magic” respectively.
Not exactly an answer to your question but a confirmation that something being magically fixed isn’t unusual.
I always go by an old professor’s law - Goldberg’s Law, which states:
“Murphy was an optimist.”
I’ve heard a term for the OP’s situation, but I can’t remember what it is. Double drat!
On the other hand, that thing with the sandwich and the jam, or the toast with the butter: that’s not Murphy’s Law, that’s physics.
When a piece of toast (substitute sandwich as you like) slides off the table, one side slides off first. That side starts falling first, imparting some angular velocity to the toast before it departs the table completely. It turns out that, for the (narrow) range of table heights common among purveyors of toast, and the (fairly broad) range of velocities at which the toast is likely to depart the table given an accidental nudge, the toast will make a rotation between 90 and 270 degrees – a range which leaves the buttered side at a distinct disadvantage.
You can try this at home; it should work with similarly-sized and -shaped objects. Give the test subject a nudge off the side of the table and note which side lands down. Do so for a range of nudge velocities. It’ll end up down-side up for a large range, and fairly consistently.
So one solution my physicist friend suggested when faced with a slippery slice of sandwich is to give it a whack. It’ll go farther, but with less angular velocity it may just come to rest preservatives up.
I did a Google on “Willoughby’s Law” (as suggested in Monty’s post) and found about ten webpages confirming that this is indeed the law that dahood is looking for.