I have a 95 Chevy Astro minivan duckbill 4.3 liter in it and the speedometer jumps and fluctuates sometimes you go up the store today don't start loss of power

the speedo problem could be the instrument cluster itself, or the speed sensor on the transmission. as far as the no start, you are going to have to check for both spark and fuel. I am assuming that your tuneups are up to date, I had a 96 s10 with that 4.3 and it had spark and fuel but would not start, ended up needing a new cap and rotor. there was spark, just not enough. if that is all good, it wouldn't hurt to test your fuel pressure to see if it is within specs for your vehicle. Btw, check and make sure all your battery connections are good and tight. bad ground can cause lots of problems.