Handling problem - bike "rolls" out like it's on marbles

[Charlie-brm]

I'm searching everywhere for clues. This issue is not commonly defined like some handling problems but someone else nailed it as handling like it's on marbles. The bike feels like both wheels want to roll out from under me to the same side. So if you picture an axis running through the headlight to the tail light, the bike stays aligned down the road but slips to the right or left. Riding it, you'd be initially mistaken that it's a wicked cross wind but it affects the wheels, not me. And it happens anywhere at any direction, not wind related. Very creepy feeling but I managed to limp it home twice.
For example, i feel as if the wheels want to both drift to the right and the bike goes to that side of the lane. I would normally make inputs to bring it back to the left but that means the bike would start leaning more to the left and aggravate my situation. So I have to gently nurse it back to the left by slowing down, staying upright and easing the handlebars to steer left instead of counter steering and using a lean.

People trying to help are guessing wheel bearings, steering bearing, tire pressure, brakes dragging, loose fork pinch bolts... I am going through all those suggestions however, the simplest explanation is usually the correct one and this bike was running great last season and I didn't do any servicing or removal of wheels or brakes over the winter. So what's different now? One clue is that it seems to crop up after about 15 minutes. I was thinking a dragging rotor getting hot but if I keep off the rear brake, it's cool to the touch when I pull over.

This is a 1979 Yamaha XS750 Special with 2 front rotors, one rear rotor, shaft drive, single piston calipers, weighs 525 pounds

[fastar1]

First thought is rear wheel bearing.

Maybe swingarm pivot?

Either way sounds like the rear tire is steering the bike.

[murf]

Was the chain adjusted? If yes then the rear wheel may notbe aligned with the front.
If not that, what air pressure in the tires did u have over the winter ?

[frekeyguy]

I think things like this...you gotta let an expert ride your bike and feel out what part might be an issue.

I am thinking more that its a tweeked frame or tires.

[autodocjr]

Was the chain adjusted? If yes then the rear wheel may notbe aligned with the front.
If not that, what air pressure in the tires did u have over the winter ?

No chain adjustment. Shaftie. Hard call. If you question it's safety you should have it looked at

[DJM]

I'll second rear wheel bearings and swingarm pivots. Let us know how you make out.