I have a 95 s-10 blazer that i've been working on for a while. i have replaced map sensor, throttle positioning sensor, egr valve, and 1 of the o2 sensors. It was running fine for about a week and now the " check engine " light is back on and it has loss of power until it hits 30 mph or higher. Then it seems to run better. Any ideas????

 

First check out the reason for the Check Engine Light. Get your hands on a code scanner and start there. It sounds as though you've had your share of emission system issues. If the Catalytic Converter has gone bad it can plug the exhaust flow through the cat., and also the catalyst inside your converter could have broken into pieces and traveled to the muffler where it could be restricting the flow through the muffler as well.
From underneath you can use a rubber mallet or the side of your fist and bang on the Catalytic Converter. Sometimes when they go bad you can hear the broken pieces rattling around inside, you can try the same with the muffler. Restriction in the exhaust system could be a possibility for the loss of power at low speed. It's worth checking into. Good luck.