There are electrical meters on the market which can measure the starter motor draw..
If it is excessive, the motor will not spin as fast - in hot weather(last month), it may not turn over the engine at all.
The battery must pass the load test; the alternator, the charging test.
Then the cables and connections must be serviceable.
Sometimes an upgrade of the posi and neg (thicker) cables will do the trick(more true with VW than Volvo or Saab).

Starter motors can last 8 to 18 and more years, but batteries , an average of 4.

The dealer may want $600 for a motor, but there are men who rebuild these for much, much less.

All the cables and connections and terminals must be "tuned up" every so often. I must do this with the '96, but the '88 only needed the ends cleaned once or twice..