Car starts and idles but has no compression in gear.

Got this car (96 Millenia with 2.5) a year ago, ran fine until about a month ago when suddenly all compression was lost while driving. Car will still start, idle in park, and rev in park (you have to feather it a little to get it going but I can redline it), but as soon as it goes into gear my RPM drops under 1000 and when I push the gas I lose everything and the engine starts to shudder like it's going to stall. It can get rolling and pick up speed, but I still can't use the accelerator. Took it into a shop and they told me it was a bad distributor causing a misfire, so we had them replace it and the car ran like a champ for the past month. Today suddenly exact same thing happened, and the shop is closed till Monday. Any advice at all? Was it ever the distributor to begin with or is this something more serious? As a side note the only issue I can find with it upon visual inspection is that on the side of the engine with the belts there is a plastic housing that covers the engine block. The seal that appears to hold this housing on is bad, I can spot a couple gaps in it. Would this cause the problem (maybe bad vacuum seal?), Or does it have nothing to do with it? I attached an image of this just to clarify, I'm not even sure if that is just a housing or something else.