I need some help, and I am hoping someone can give me an idea of what might be wrong before I take my car into a shop and get taken to the cleaners like I did the last time.

I have a 1998 Sentra GXE, and here is what is happening.
When I excelerate from a dead stop, my car will bog down, I hear an engine rattle, a white puff of smoke will blow from my tail pipe, and then my car will slowly begin to gain speed and I mean slowly. I can have the pedal down and it won't go any faster, seems to take about 30 seconds to get up to 25mph or so.
The most curious thing, is this is only happening about once a day on a 12 mile trip and happens onlyin the afternoon, it might happen twice a day if I am driving more distance. But where it happens at one stop light, it doesn't happen at the next. My temperature reads fine and the check engine or oil lights do not come on.
The air temperatures here in the morning are still cool, around 60-70but we are hitting about 101 in the afternoons.
I would appreciate any help I can get with this matter.
I have one person telling meI have a bad sensor
One shop telling me I need my engine seal replaced (10 hour job)
One person telling me my converter is going bad.
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thank you for reading and thank you in advance for any advice.