All good advice in the previous recommendations. I shimmed both wheels with brass thrust washers to get them on the same plane (checking with a long straightedge spanning both wheels) also, the washers minimized the wobble of the free wheel.
DoN's comments wrt the back edge rollers.. definitely worthwhile to check those.
Safety first.. I removed the worm shaft pulley and installed a long tap handle to manually rotate the wheels and band slowly while adjusting the pitch of the free wheel.. an adapter for a slow running variable speed drill would serve the same purpose of running slow (with full blade tension) while making checks. Using the saw's motor to run the blade while making adjustments (or any time the blade guard is off/open) would be a safety risk worth avoiding.
The powdery swarf sounds like worn blade teeth, you should at least see fine sparklies for chips with a fine-toothed blade, and of course larger chips with a coarser tooth pitch. You'll get used to the prickly feel of sharp blade teeth on your fingertips, but a magnifier may reveal small flats on the tips of the teeth you're using now, indicating a worn blade.
Also, after correction of the wandering blade travel, it's important to avoid too light of a downward pressure (while cutting) on the blade.. rubbing without creating chips will dull the teeth quickly, so try to maintain at least moderate downward pressure.. more pressure is good when the chips are plentiful.
You may have noticed it mentioned before.. a good cutting lubricant is very cost effective and greatly enhances cutting performance. My long-time favorite has been Lenox ProTool Lube.. just scored several 6 ounce bottles on eBag at $3 each (Fastenal no longer stocks it).
Lastly, avoid cutting OBF old bed frame.. the hard spots will wipe away the tips of the blade teeth immediately. If necessary, it's better to cut OBF with a cutoff disk in an angle grinder, or use a hacksaw to avoid the need to change the bandsaw blade. HSMs would be better off avoiding it, since it also wipes out cutting edges on drills, and it doesn't weld worth a damn.. it'll look like it's welded, but the weld is likely to pull out with light to moderate force.