1999 Int. 4700 T444e no power under 50 mph

Former Emergency Management command center. Always maintained but sat for 2 years. 275,000 miles Starts right up and runs fine except that any uphill grade from a stop or slow roll is painfully slow and labored. Once it hits 50 +/- it will scream up hills. Fuel filter and screen are new. Oil is clean. Any ideas?

 

Thanks. I checked the main pipes and hoses and they are all in good shape. I'll check the ones on the back of the heads. What's the best way to check the charge air cooler for leaks?
It does have the Allison trans - the only one I've ever seen with "Park" - that when selected automatically applies the air brake.
I also have a 1997 4700 T444e with a manual trans that doesn't suffer from excessive power but never makes me think something is wrong.