Jumping and knocking
 

So my fiance and I recently bought a 92 grand marquis from a friend of his. To say the least, me and my fiance know nothing about cars. This whole deal may be a tad... or a lot nieve of us but after he had already paid his friend and was leaving with the car, his "friend" notifies him that after you go past 40 it makes a sound. He assured him it was not the trans he got it checked and it was nothing. He did test drive the car, but not going 40. The first time it happened to me I called my fiance and he insisted it was okay because his friend said it was. I myself thought it was pretty odd. What it was doing was this... once I hit 35,45,55,65 mph ( i was on the freeway) the car would slow down, take a very long time to pick up and make a rattling noise. Well he insisted it was okay so we've been driving it for the past few months just ignoring it basically. A few nights ago I was driving home and the car started to shake really bad and started jumping for about 30 seconds and then stopped. Then I've driven it a few times today and it has gotten way worse. The jumping has become rough, and accompanied by a very loud knocking sound. When the car starts to settle down (like at a red light) it goes down to a light jumping and vibrating and instead of knocking its that rattling sound again. I'm no expert, believe me, but my assumption is whatever this issue is, is the same issue that was causing the rattling but has gotten worse. I'm not driving it at all now, I'm scared to death it's going to break down on me. And trust me, it takes a lot for me to say that.. I'm not one of those people that thinks their cars going to break every lil sound or quirk it has.. i'm pretty laid back about it, actually too laid back about it. So for me to say that it must be pretty bad. I was just wondering if anyone had any advice on what it may be and the normal ballpark range of price. We're pretty broke right now going through a real rough patch and was wanting to have ideas of how much it may cost to fix so i know how long I'll be out of a car. Thanks in advance :)