Gopher,
I own B-50 spools from different years (almost ages) and not all are the same - as far as applied wax, initial twist (spun) and surface characteristics. Some are rather hard and/or slick whereas others are soft and lean more. They just don't mix in a flemish string and would produce something you described.
Good advise was already give above, but let me add some thoughts. Make sure you:
- align strands parallel and don't change start and end positions, after cutting them off the spool
- twist every bundle in the same direction - always tightening the initial twist of each single strand. If some start to fray you've mixed ends (see point above). This will also increase diameter
- pinch each bundle between thumb and index finger (or what else boddy appendices you like to use) - so parts already done don't move, as you revers twist loops and splice
- make sure to apply same amount of twist and stretch as you go
- if it doesn't work otherwise, then give a good deal of wax to each bundle to make them act the same. I prefer less wax though, but sometimes this is the way I've to use
- try a three bundle string. Even though it's more difficult to keep all tensions even here - it might be easier to do for you?! I like them better.