I know what you mean. Our first R75 on an early 51 plate bought from a dealer in 2005 absolutely ate bulbs but I could never find an issue. Our ZTs and our second R75 on an 05 plate have not had any of these issues.
The first one was a v6 and the second one a.diesel so different alternators but both ZTs are/were 190s and they didnt suffer..
Initial thought would be alternator over charging but I never saw that. I just put it down to an electrical gremlin I could never find on 6 years of ownership
Other members might be along in a bit who may have found the issue
__________________
Jon C
Living the dream
MK IV VVTI Tiptronic TT Toyota Supra MG ZT 190+
BornTuesday, 10th July 2001 @ 09:01:10
259th ZT to run off the production line, out of 18,894
252nd ZT 190 + to be made out of 4,023
101st ZT in Solar Red (code: CEU) to be made out of 1,888