1991 Pontiac Firebird turns over but won't start

Engine Mechanical problem
1991 Pontiac Firebird 6 cyl Two Wheel Drive Automatic 189k miles

i bought this car about a week ago, absolutely fell in love. It drove just fine for about a day and then it started to bog down and would go a maximum of 20-30 mph give it gas just wouldn't go faster than that, so we thought there was seriously something wrong but we drove it a couple hours later and it was back to normal.

the next day it started up. And then died. Turned it over and over it wouldnt start. Waited about 15 min it started up took it for a drive and it started to bog down again. So we parked it and planned on taking it to someone to figure out what the deal was. Now it won't start at all but it will still turn over. We've replaced the fuel filter, checked the air filter it was fine, checked the fuel pump its still running fine, & we've replaced the MAP sensor, it appears to have brand new plug wires. Still won't start.

do you have any ideas that could be wrong that we've missed?

(we've thought about replacing the spark plugs but I kinda want a better idea of the problem 1st)