Tramline,
You don't have to put all 150 Mode S codes in at once - you can just do a few at a time :-)
If it is aircraft fleets you are following, can you use wildcards in the Registration alert box like I'm using in my Fleet Watch setup? This list give me all the BAW B747/777 and all the registered A380's and the Beluga fleet.
A6-ED*
F-GST*
9V-SK*
G-CIV*
G-BNL*
G-BYG*
G-VII*
G-ZZZ*
G-YMM*
G-RAES
G-BRZS
VH-OQ*
I don't know if you have used airframes.org, but you can search for operators as well as individual aircraft, so you can have a list of all their aircraft and just click on each one to see the Mode S code. Often tese run in blocks too, so again you might be able to use wildcards.
I think that it's possible that you have so many alerts set, that RB is just being maxed out - maybe you are getting 2 or 3 alerts at the same time?
As I've said before, although it doesn't help you, I use the alerts function extensively and also the Fleet Watch facility (in fact I use this more that the alerts now, as I can double click on the aircraft in the Fleet Watch list and go straight to it) and it works as advertised for me.