98 Honda CRV Hesitating/Sputtering when steady on gas

Hello

Forgive me I do not know much about cars but I would like to learn as much as I can, I know this is a good car for my needs and I need it to last me as much as possible. With that, here is some back ground info.

This is my first manual transmission which I purchased 8/09 with 187K, it now has 222k. I have had the timing belt changed, and serpentine(sp?) And other than that, just regular high mileage oil changes and haven't had any other engine troubles. In January I began making a 60 mile drive to work, which has about a 3,000 ft incline in the last (or vice versa) 20 miles or so. I noticed I had to start down shifting more often about 5k ago, added fuel injector and noticed a small difference. I have also noticed that it can sometimes be hard to put into gears, including reverse. Recently I let a friend borrow the car for about 2 weeks. When she returned it she advised the engine light had been on for a few days and it had started doing this hesitating/hiccuping/sputter. I notice the issue not when I am accelerating per say but just when I am steady on the gas. If I am going up hill or it seems when I am at the higher end of my gear, I do not experience the issue. I felt like there was a bit of a loss of power when I began driving in the mountains, but I assumed that was to be expected. I am wondering about burnt valves possibly? Would just a simple tune-up be appropriate? Or necessary? I have never had one yet. Should I pay for a diagnostic test for the check engine light? The mechanic I went to today quoted me $350 but I am not sure if that was for Honda parts, which I am going to inquire further on. I just paid him $130-$50 for and oil change and $80 for rear differential. He told me $120 of it would be this 3 part fuel injector/ valve cleaning kit. Does this sound legit! Sorry so many questions, just moved from my reliable friend of the family mechanic and don't want to be taken advantage of!