Blazer cranks but wont start

For a couple weeks now the blazer would start but was difficult to keep running, if you press the gas it acts like its gonna die but if you release the gas it will idle fine until you put in to gear then it starts over again for about 2 minutes then it seems to be fine until you stop at a stop sign as you take off it is very sluggish and cuts out then its fine again. A few days ago it quit starting all together, went and bought some starter fluid sprayed into air intake and it started right up but wont stay running. A couple of times I was able to rev engine and hold to floor but would then die. Any suggestions to my problem. Thanks so much.