Bucking at 50mph

When I first got my truck I noticed it bucking at around 45 mph at random times and it only last a couple seconds. For the past two days I’ve driven it on the highway after letting it sit in the hot sun all day and I can’t get up to 55mph without it jerking so bad it feels like I’m about to break down. Then it’ll stop and once I let it sit for a little while it’ll drive fine the rest of the day. Check engine light has only come on once and then turned off after a couple of minutes. Anyone know what could be causing this?

 

First guess is torque converter slipping due to clutches. Auto or manual trans?

Second guess driveshaft unbalanced?

 

@fajitas21 auto

 

Auto supports my TC theory. Have you recently changed the transmission fluid?

 

Start by changing the spark plugs.

I had three 8 year old F225 4 strokes Yamahas on a 42 Performance and they would not get over 5000 RPMs when they should have tacked at 5900 rpms. Had these engines since "new".

I changed the plugs and got 5900 RPMs fast and smooth.