/6 problems

I had the motor rebuilt about a year ago. Has about 1000 miles on it, daily driver and only to work, school and home. Was running great.
A recent trend has now been this, I start up, switch into reverse start backing up and either die or I put it in drive and start to acc. and it dies. When it doesnt die, I cant really get it to acc. very well at all.
Any ideas?

225 bored .30 single barrell holley, 904 and typical rear end. The fuel tank was never cleaned or switched out so I was thinking maybe it could have something to do with that or the tranny.