94 Blazer wont start cranks but doesn't "catch"

I have a 94 chevy blazer s10 with a 4.3 engine with 154,000 miles on it. I recently had a tune up done but it still ran rough and would stall out after driving when I would try to stop. Was told oxygen sensor was dead and was going to have it replaced but now the truck will not start. We crank the motor and at times it tries to start but wont catch completely, it did catch a couple of times but I had to press the gas pedal all the way down for it to "take off" then when I release the pedal it runs rough and finally will die. It is injected and you can smell fuel when it cranks. It is getting spark also. Also after sitting for a day or so it will usually catch and run (roughly), when I was driving it I had to keep my foot on the gas and brake to keep it running at stop lights. What can possibly going on? Where should we start to look to fix this? No codes come up when we check (except for oxygen sensor) so we are at a loss. We can also hear the feul pump when we turn key to on position. PLEASE SOMEONE HELP! THANKS in advance! :)