Engine stalls when 98 Protege placed in gear

I have a 1998 Protege LX with 156,000 miles, yesterday I started it and it started just fine. However, when I shifted to reverse and took my foot off the gas it shuddered and then died. I restarted it and it did it again. When I restarted it for the fourth or fifth time, I put my foot on the gas and it seemed to do better but when I would shift to drive it shuddered and died again. I idled sporatically in nuetral but did not die. When my husband looked at it I put it in reverse and it died. So I restarted it backed up, went forward and then put it in park so he could listen to it. As he was listening the idle evened out and it stayed running when shifted to different gears. When I drove it today it started to shudder but when I gave it gas it stayed running. It was really slow to start gaining speed when I went down the street, but I kept giving it gas and it kept going. Eventually it evened out and I had no other problems the rest of the day. This has us both stumped as I have not had any problems with my car yet. The transmission was flushed and had new fuel last year, I also replaced the fuel filter within the last few months. Do you have any ideas as to what could be causing this problem and how expensive it is going to be to fix.