You may be suffering from the negative getting warm under the heat of the lamp and then 'popping' into another plane of focus. The only long term satisfactory answer is to get a negative carrier which has at least one half fitted with glass covering the top of the negative. This glass must be the 'Anti Newton Ring' type or you will get concentric, oddly shaped circles, appearing on the print. If you get a carrier that has glass top and bottom then there there will be no problem with ring from the bottom glass. However I have always found a double glass carrier seems to attract more dust.

My enlarger, a LPL6700 has a glassless carrier for 35mm and that same problem was solved by fixing a Anti Newton ring glass from a 35mm transparency mount to the upper half using double sided adhesive tape. Every now and again I remove it to clean properly, refitting it with a new piece of tape.

Most medium format (and above) enlargers have the glass carriers fitted as standard with ANR glass already in place.