Regarding the inability to get the bike to hold the 11 tooth cog, check that your high limit screw is back out far enough. But more importantly the 53/12 gives 32 mph at 90 rpm. Are you trying to go faster than that? Is your cadence much lower than 90. Surely you should have use of your 11-tooth cog, but you should also be able to easily spin at 100 rpm and a normal cruising cadence of 90 is considered by many to be close to ideal. Perhaps some work on both things is in order.