1997 Dodge Ram 1500 weak start when warm

So, whenever I go to start my truck after it has been sitting in the sun, It
starts up very weak. When I turn the key, the starter makes it's usual sound
(sounds normal) but then when the engine actually starts, it is a very slow,
weak and quiet start. It just slowly sputters up to its normal idling RPM and
then runs just fine. If I start it early in the morning before the sun has warmed
it up, it instantly fires right up without a problem. Its had this problem for long
enough for me to know that it is directly related to whether or not it has been
sitting in the sun. I don't think its the battery because it was replaced a year
or two ago, and I had this problem even after replacing it. And like I said, the
starter sounds like it works just fine, its what happens after the starter does
its job, when the engine ignites that is the problem.