All was wonderful until I changed the upper engine mount. About 50 miles later the car started to run very ragged, first at idle only, then at all speeds including acceleration.
It feels like it is running on 3 or 4 cyl instead of all 5.
I rechecked my work - nothing detached, bent or broken. I re-sealed the intercooler tube on the back side of the engine just to make sure I hadn't messed that up.
It ran better for about 5 minutes then went back to the ragged running.

I've had this car for about 5 months and it has run great until now. What did I do wrong? or is this just a fluke that has nothing to do with what I did? Any ideas on how to fix it?

 

Could be a fluke, is the check engine light on? if so I assume you don't have a scanner to check codes, you can have it scanned for free at many local auto supply stores, you may find out that you just have a bad coil or something completely unrelated to your job. More info and I will come back to help you.

 

Check the Electronic Throttle Body. They get gummed up with residue from the crankcase ventilation system. They should be cleaned about every 75,000 miles. After I had that done on my 2004, the engine ran like new. There are some sensors that measure the position of the throttle plate that can wear out and perform erratically. Also, install the original Volvo spark plugs; that always seems to help. You should also check the coil overs because they can fail and cause misfires and dead cylinders.