Cisco Switching/Routing :: 3750 - MSTP Design And Network Diameter Limitation

I am just browsing and looking for a solution to converge my multi-vendor switched network and bring some redundancy to it as recently
we managed to get a redundant links. I have a need to change core switch to Cat3750G, which has Per-V LAN-RSTP+ on board, but tests have shown that it won't be compatible with some other proprietary per-V LAN RSTP solution other vendor's switches use currently.
 
So, I thought maybe standard-based MSTP design might do the trick. I've made some tests and got some weird and unstable switching result. I have two topology rings with a core switch in the center. Every ring has about 10 switches, so practically network diameter may vary from 5 switches (when spanning-tree converges in the center and I have a blocking port somewhere int the middle of the ring) to about 10-11 switches (if a I have link failure on any of ports right at the core switch). I disconnected one port from core switch to eliminate a possible switching loop while I will be configuring new MSTP design. Then I started enabling MSTP on all the switches staring from core Cat3750G to MSTP, one by one, placing all switches to the same MSTP region, and placing all V LAN's to default MSTI0(CIST) cause I don't need to organize any separate MSTP instances for every V LAN or for group of V LAN s. When I turned MSTP on on 7th or 8th switch in the chain (cause I had a physical chain when I disconnected one port out of redundant ring) I got all switches "flapping", storming and flooding the network with broadcasts. Even when I had one redundant port disabled.
 
I have no idea what I am doing wrong. I noticed that Cat3750G has an option that defines a possible network diameter which actually automatically changes some hello, max age etc. attributes according to diameter specified. When I defined a maximum network diameter of 7, if didn't change anything: I still have hello timer of 2 sec etc. I've been wondering if the maximum network diameter has something more than just a "variable" to fine tune hello timers etc? Maybe I won't be able to use MSTP in my network which might have diameter more that 7 switches. Or maybe it was a mistake of placing all the switches to the same region and all the v LAN s to the default MSTI0 (CIST) and I should configure one MSTI per V LAN or per some group of V LANs and subdivide my switches to few MSTP regions?

My topology briefly looks like this:

+--SW1----SW2----SW3---CORE---SW4---SW5--SW6---+
|                                            |           |                                         |
+---SWxx---SWxx-----------+           +------SWxx-----SWxx----+
 
As I said, each "ring" has about 10 switches connected side by side.