Losing power 4th and 5th gear

To whom it may concern, I'm new to this forum as I just purchased a 79 z650 a few weeks ago. I'm having an issue I was hoping I could get some insight on. The bike runs great overall but occasionally I'll be cruising at highway speeds (60+mph) when all of the sudden the bike starts to lose power as though it's starving for fuel or Ive lost a cylinder. Getting on the throttle harder has no effect and if I squeeze the clutch the rpms plummet to idle speed. This doesn't happen all the time, only after I've been riding for a few hours or near the end of the day of riding. Otherwise the bikes solid in all gears and speeds. I suspect I may just need new ignition coils since I'm not sure they've ever been replaced. I've read that excessive heat can cause irregularities in the performance of the coils but I'm just looking for some other options on the issue. If not the coils then what possibly could be the issue? Thanks a ton, Pat.

Welcome. The first thing to do is check the valve adjustment and a manual. The valves are neglected on these bikes because it takes time, work and money to do them. Not much but enough for lazy PO's to just sell the bike. You will check the spark advance to see if it is free and lubed. Depending on if you have points, you will adjust them as well. Ruling all these things out, you can start thinking about the coils and or carbs.
Steve

Thanks a lot man. I'll see if I can get my hands on a manual and start wrenching.