If it happens when engine is hot then relay mod will be your cure. Strangely I have had a similar problem with cold starts that has been getting worse. Took the starter (also 23 years old) apart and cleaned it but no better. Had an auto electrical company look at it and they declared it ok. Cleaned every contact and replaced some cables and bought a new battery but still no improvement. Borrowed a starter off one of the other LADS and immediately it turned engine over much quicker. Problem gone. I guess after 23 years my starter was just "tired".