2003 Mazda Protege Poor Acceleration; Almost Stalls

I am original owner of:
2003 Mazda Protoge
109,000 Miles
Have always run 87 octane fuel - until recently (see below)
Oil changed per owner's manual; at 85,000 miles started using Mobil One High Mileage.
At 95,000 miles (summer of 2007) had the 100,000 major service done - at that time serious rattle coming from engine. Had timing belt adjusted; replaced water pump; had valves adjusted.

Current Problems:
1. Check engine light comes on some days steady, other times it doesn't come on at all. Had code read by Autozone.
2. Loud rattle has returned - volume of rattle increases as I give car gas.
3. RPM's drop very low then pick back up when car stopped. Feels like vehicle may stall.
4. When accelerating from a stop, e.G. Traffic light, the vehicle has a very noticeable hesitation.

For the past month I have run 93 octane fuel in car. I have also put in STP Fuel Treatment two of the times I filled the gas tank. Seems to make engine run smoother, rattle is much less or almost gone. But the problems have come back.

Should I get my fuel injectors cleaned? Valves adjusted again?

I would like to keep this car for at least 5 more years, but don't want to break the bank.