Starting woes

[zed4life]

Greetings all. I've just bought and am restoring an 85 TX5 Ghia for my pensioner father and am noticing an intermittent starting problem.

Symptoms are identical to a flat battery in that not enough juice to starter ie: starter turns very slowly on some key turns "rrrerrerr" however battery is @ ~12.5 when ign off and 13.8 on so battery and charging system OK. Other times engine kicks over freely and starts/idles/runs perfect.

I suspect a dirty connection / failing starter motor/solenoid.

Have :
* cleaned/tightened the battery terminals;
* tried bridging the 12V and starter wires @ ign switch (but can still induce the problem)
* am cleaning the starter motor wiring terminals next then
* replace the starter?

Q: Are the starter motors/soleniod and/or ign switch in these a weak link or is it more likely dirty electrical connections?

EDIT: PS: Are there any wiring diagrams/online service manuals available?

thanks