Cold Machine Operation

[prh77]

Original 1988 installation. While running, unit will lose rpms- judging by ear- about every 45 seconds. Does not stop...just slows down slightly and then picks right back up. Will do this slight cycling at 45 sec intervals like clockwork, so I do not think it is a fuse or wiring issue as it is too regular. Not sure if this is new, just began to notice it this week.  Any thoughts? Seems to cool fine, although it is still pretty cold up here.