Chevy 8.1 Engine Issues Again!

I posted last year regarding our Chevy 8.1 Workhorse engine going into limp mode, well it happened again today while pulling a hill and towing our toad. Prior to it happening, our rig just acted like it had little power. The gas sat through the summer, with Sea Foam, is it possible that this could be the problem? Any ideas, experiences and/or advice is appreciated as always!