Chevrolet: power loss, good shape, serch

Question
I have a 1983 G20 van with a 350 engine. It has about 175000 miles. The van is still in good shape and runs good except at times it does not seem to have any power. I use the van for construction work and with a load (even a light load) it does not want to pull. I can floor the accelerator and may get 35 MPH on an incline. At other times it does just fine. A y help would be appreciated.

Answer
Tom;
  Hi - well it looks as if it has a lot of miles on it , you can try a tune up , but I would look into a rebuilt motor for it , as you say the rest is in good shape. the rings and cyld. might be worn and the valves also. The 350 should pull a load ,so you can look at the rebuilt engines for it ,a good turn key motor would be  good for it.  
if you do a serch for rebuilt motors on the web you will find a lot of them - ok