Mine did the same thing. I bought one of those fancy timing pointers to replace the coat-hanger special I used to set TDC and forgot to tighten the bolts that held it to the timing cover, one of which replaced a shorter bolt down near the
oil pan. It drove me crazy for weeks. I even crawled under the car while it was running (not recommended for the sane!) but couldn't spot it. Finally, after removing the pan and most of the front of the engine I discovered what I had done. I wouldn't talk to myself for a month.
Bill