I have a 1989 883 sportster and at highway/interstate speeds (first showed up at speeds over 70mph, last week at 55mph) it cutts out just like it is running out of gas. If you role off the throttle about 5 or 10 mph it comes back to life. As long as you stay around that speed it you can usually keep going, but if you try to get back around the cut out speed it will happen again. I thought that there may be a gas flow problem but it does not seem to be the case. The local dealer thinks it may be an ignition problem. oh and by the way, temps here in Louisiana have been in the 100s but it started before that.

Drain the gas out of your carb. There might be dirt in it. Same thing happened to me.

The first thing I'd check is the fuel flow. Pull the fuel line off the carb and turn the petcock on. Watch the flow of fuel coming out of the hose to see if it's a good flow. Next, what type of exhaust system is on the bike? If it's got open drag pipes on it, the engine probably needs a bit more back pressure. The old Ironheads were notorious for having a "dead spot" at about 55 mph with open drag pipes. Most drag pipes have a hole on the backside for a bolt that held the baffles in them. If you have this hole, get a quarter inch diameter bolt that is about an inch and quarter long. Put the bolt into the pipe and thread three nuts on the bolt inside the pipe. Make sure the nuts are tight. Put one in each pipe. It's a cheap fix and works pretty well without changing the sound of the bike. Also, at about 55 mph, is where the carb changes over to running off the main jet in the carb. Your carb could be jetted wrong or have a piece of trash in the main jet. If the bike is running rich, it will run better with the air filter removed from the carb. If it's running lean, it'll run better if you roll back on the throttle like you said. I'd check all this stuff before I started buying ignition systems. That's an electrical part that the dealers cannot test and once you buy it, if it ain't the problem, they won't take it back. Harley Davidson ignition systems are pretty rugged and I've never seen a lot of problems out of them. That would be my last effort.

Good Luck
Steve