I actually took the car to him to have the work done, even though he usually does mobile work. (He's a lot cheaper if you come to him and drop it off.)

I'm not sure if he'd be willing to drive out to me without charging a trip charge, just because I have a concern his work might have caused my new problem....

That's why I thought I'd run it by people here who might know more about it. Again, I'm not seeing any oil leaking anyplace, and I had a few occasional, random issues with the car's RPMs suddenly dropping and making a quick shudder when slowing to a stop at a stop light or sign, before I had the gasket repaired. (I was advised that those "hiccups" could have been caused by oil seeping down and getting onto a spark plug -- hence motivation to get the gasket fixed sooner rather than later.)

If this turns out to be a vacuum leak or something, I'd rather not get stuck paying him to diagnose and/or fix it and bill me. But if this sounds like a case of, "Yep..... he screwed up. Been there, seen that." -- then I'll call him right away!