Thanks again everyone for the replies. I don't think it has anything to do with the enlarger light or electrical current for a number of reasons. In fact I'm convinced that is not the problem. First I have a voltage stabilizer for the power going right into the enlarger. While I can't guarantee that it works properly I have no reason to believe it doesn't. My enlarger uses a halogen bulb so that shouldn't be a problem. Most importantly within an enlarging session I've never experienced anything that would indicate a problem with the electrical current or the bulb. The darkroom has been set up for a little over a year. With hundreds of exposures when I make a change in exposure (within a session)the result has never been inconsistent with that change, even though many times I'm not fully satisfied with my decisions. Certainly nothing comparable to the sometimes 25% change when I reprint from established exposures in a new session.
After I'm satisfied with a print often I will make 3 or 4 copies. I'll expose develop,stop fix a process that takes around 4 or 5 minutes and go on to the next copy. I've never experienced any noticeable differences between the copies. Just last week at the end of a session (2am) I noticed after toning and drying one of the prints was too light because I misjudged dry down. So the next morning with the same chemicals I adjusted the exposure about 15% and the change was right on. Made 3 copies and they were fine also.
So it must be something else. I mix up Dektol from quart or 1 liter packets maybe in the smaller packets the chemicals aren't as evenly distributed than the larger packets. Maybe Kodak's quality control isn't what it used to be. Whatever the problem it is a real PITA when I have to duplicate a print in a new session and almost have to start over.