88 Jimmy Sierra Classic engine problems

I have an 88 sierra classic with a 5.7 350 v8 fuel injection engine and after it warms up and come to a stop then try and push on the accelarator it hesitates bad like it just soen't want to go then after a few times of doing that it will pick up and go,but then when I stop again I'm back in the same situation. I notice that I have a loose valve and have tighten it but it's chattering again. Do you think that it might need a tune up, injection service or any other ideas. besides new motor..it does have 98700 miles on it. It does exspell a bad oil burning smell.. any thoughts would be helpful..thanks.