Oh, okay..... I had just assumed that the engine was a later thru hub exhaust model. However, you say that it is a electric shift model which brings up another type problem.
BTW, it's unlikely that you would see evidence of a slipping hub in a propeller just by looking at it. You can make a mark on the hub and also on the prop, having the two aligned, then run the engine. After the problem surfaces, inspect those two marks. If they are no longer aligned, the hub is slipping.
Having said that..... Those old electric shift models were notorious for having the shifting hub slip within the torque spring within the lower unit. These two items are very expensive articles if you can find them.
Make sure that you do have a full 12 volts being applied to the lower unit (green/forward) shift wire.
Check the lower unit for gearlube. Make sure that it is full of the proper lube which is "Premium Blend" also known as "Type C", available at any Evinrude/Johnson dealership.
With the gearcase full of the proper gearlube, and with a known good propeller, if the unit still slips, you are then unfortunately faced with a lower unit repair or replacement job.