Swift kick to the nads needed to start hot truck

[bigcarguy]

When my truck is cool it is easy for the engine to turn over. The problem is that after I drive it around for a while, it can barely turn itself over. I thought maybe the alternator was bad, but the truck cranks fine when it's not hot. A friend of mine said that the starter might be my problem, but I put a new starter on there sometime last year. We replaced the starter, but we kept the old solenoid because the parts place gave us the wrong one and we didn't have time to go back. My question is this, would getting a remote mount solenoid from Summit do the trick, or do I just need a new solenoid? The remote mount solenoid is only like $25 so it wouldn't be much, but I was wondering if that'd do the trick.