300E is hopeless in the hills!

We love our 92 300E, except when we climb hills, like around the Sierras, when the elevation gets above 4-5000' and more. Unless I have a big run on the hill, I eventually am doing about 25 or 30 mph with a line of cars behind me. It's embarrassing. In town there isn't a problem. Even if I floor the accelerator on the hills in the mountains, nothing changes, no down shift (unless I put the trans in 3rd or 2nd (when it's real slow).
I realize that it's only a 6 cyl, but this seems rediculous. We use only the best gas, mileage is around 24-25 mpg., no overheating, no trans. slippage, no nothing. I even had the injectors cleaned, thinking that might solve the problem. Do I have to learn to live with this or is there a solution?