I wouldn't be surprised when it starts to warm up where you would be shifting into 3rd up to 5th with the increased demand, your Ignition Control Module is failing. You might not be getting enough spark which would cause that condition. It's probably due to be replaced. However, it could also be caused by the carb. If a cleaning and adjustment doesn't work I would replace the ICM at about $70. It's located under the coolant expansion tank. Plug and play. Easy to replace. Sputtering, bucking and loss of power usually means the ICM, thinking more about it.My 89 YJ 6cyl manual looses between 3&4 and 4&5 gears. It sputters, bucks and looses almost all power in 5th gear!! HELP!! Can anyone tell me what's wrong with this thing?