half power crank, no start

[elbo989]

Out of the blue my truck was cranking only half power sounding pretty rough. If I keep cranking it will eventually crank full power but not start. Truck randomly started two days later and the following day same symptoms. Blew the solonoid on a new starter while putting it in but the truck cranks the same as before so I dont think thats the issue. I've heard it could be Hpop fitting or plugs. Any ideas?