91Z-TheReaper

The last few days I have been experiencing some odd problems during accelleration. What happens is this: you give it gas and it starts to take off as normal, but all of a sudden, it seems as though it is getting 'stuck' - the RPM and engine sound continue to climb, but the car has no further pickup. Shift into the next gear and everything is fine. Doesn't happen again until car has been shut down and sitting for a while.
The first instance, shortly after start, occurred only in first gear.
Then another occurrence was only in second. And another in third and another in fourth. No problems in 5th. Happens most often in 1st or 2nd.

I've been in to see my mechanic (I thought I had an oil leak), he said the master rear seal on the pan. Okay, hes the mechanic. He didn't want to go after it (I've only had to add a liter in a month), because my exhaust is welded as a one piece and makes for a very lengthy job to take it off. Anyway, could this possibly be a tranny fluid leak? He didn't think so because he said I would definitely feel/hear the grind.
Any thoughts on what I may be having a problem with? Any comments would be greatly appreciated.