I've been dealing with similar gremlins on my 79 that had been sitting for years and abused before that.
What I have found is any bad connection (dirty, corroded, caked with old grease) can cause intermittent or slow operation of that circuit.
Almost all of my switches I have taken apart, cleaned and reassembled using dielectric grease. Lots of small, spring-loaded parts so observe how everything fits and don't lose any bits in the process. And easy with the housing tabs so you don't break them.
Wiper motor slowness can be attributed to any poor contact in the circuit, including the switch, wiring connectors and contacts in the motor assembly itself. Not a hard job to take it apart, clean lube and reassemble.
Also, check to make sure you have good grounds on whatever circuit you're working on. A corroded or missing ground can create similar symptoms.
Anyway, if you're up for the challenge and don't need new, DIY has worked for me.