I built up a Miyata from the bare frame. Everything worked fine for 50-75 miles. At that point the Sugino Super Maxy crankset began to slip. I removed the crankset and inspected it for damage and seeing none I reassembled and immediately it slipped again when pedaling. I used a Sunlite SL-26 cartridge BB size 68 by 118mm with 1.37” by 24T English thread. Drive train aligns perfectly.

No metal grinding or damage, just slips. Weird because it worked initially. Any thoughts? Thanks.