When I was first started on Inderal in the early 80's, there were no generics available. Once the 20mg tablets became available in generic form, my doctor preferred on stay on the "regular" brand. I'm not sure it makes a difference, but it might be worth testing it out. Can you go back to the regular version for a month or so, to see if it works better.
At one point I was taking the 80SR, but found it necesary to supplement with a 20mg tablet for breakthroughs.