Strange shifting problems...

I seemed to have developed a strange shifting problem. I'm going to change the oil this weekend, and put some fresh 10W-40 in it, but, wanted to get some insight from you guys in the meantime. I've been told an oil change does wonders for shifting problems.

When it's cold, it shifts fine. I can run up and down the gears with no problems.

After it warms up a bit, I seem to get into problems going from 2-3, or 3-4, or 4-5; only going to the next gear. It doesn't do it when going from any gear down.

I'll be going along, and when I want to shift, I'll pull the clutch, let off the gas a little, and when I go to pull up on the shifter with my foot, it feels like it's in 5th, and I can't shift any higher. If I let the clutch back out, then pull it again, I can get it to shift.

Any of this sound familiar?

what kind of bike?check shifter linkage.

Sorry; it's a 78 KZ650 B