Hello, welcome! I'm not sure what could be the specific cause of your problem, but there are a few things you can do to narrow the search. If it were me I would:
a). try a second tape just to be sure the problem isn't recorded on the tape.
b). change the playback speed, if the tape is recorded at 7.5ips try 3.75ips and see if the dropout interval decreases or stays the same. This will tell you if the problem is related to the transport rather than something electronic.
c).if the rate of dropping out remains the same regardless of the speed then the problem is in the electronics and it is common to both channels. This might point to a problem in the power supply.
I hope this is somewhat helpful.