T350 issue??

[jescates]

Drove the truck Saturday, at 50mph, truck felt like it was surging. Went to get on the interstate and it would only get up to 65, still surging. Got off at the next exit and at 50, it seemed fine. Stopped at Wal-Mart, then hit the road. Truck drove flawlessly! Put about 100 miles on it Sunday and Monday with no issues. Drove it to work today and no problems. Then on the ride home, did 70mph, but started feeling a little surge. So took an early exit, and the truck would barely get up to 40 on the back roads. Checked the fluids and it's a little above the full mark. I plan on changing the filter soon, but just wondering if an overfill would cause the surging or am I looking at possibly bigger problems? The transmission was rebuilt in 2004, but it has sat for the last five years.