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It sounds like the fluid is draining back into the trans oil pan. Check the fluid level after it sits, it should go up as it sits if it is draining back. I wouldn't put it into gear and gas it when it is acting up, that can burn the trans up. Put it in neutral and rev it lightly a few times, this should purge the air out where it will go into gear normally. You can try flushing it out, and changing the filter. Sometimes valving being a little gummed up can cause this type of problem, in that case fresh fluid and filter can help. Other times it can be from wear, and it is a problem you have to live with, or rebuild the transmission. I have seen them work for years in this condition, the neutral trick should help it survive. If you have any other questions feel free to ask.