I made a thread about the same thing when he was first released.
http://www.cybertron.ca/boards/showthread.php?t=65611
Mine leans forwards like he's drunk. I did two things to kind of help address this.
First, I put a crap load of nail polish on his hip ratchets. I couldn't take the bot apart, but I could access them by opening up Scattershot's chest.
Second, I put a weight between the nosecone and tail-fin on Computron's back.
Now he stands straight with Scrounge on his shoulder and supporting the PE add on kit's new chest piece and bigger rifle. I pegged the original rifle onto Computron's back.
In case you're wondering, the UW version has similar issues, but not quite as bad. But I think its stability has to do with it using the original dumpy combiner feet. Those feet are more stable.