1986 ford f150 windshield wiper problems?

I have a 1986 Ford F140 XLT and wy windshield wiper will turn on, work for a few swipes then turn off then back on then turn off about half way then start again. And once it starts to turn off and back on again it take a long time for it to wipe the whole windshield. I was told that it wasn t the switch otherwise they wouldn t turn on at all. And I replaced the motor and the black box under the dash. I use a lot of Rain-X in the mean time.

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