could be the starter motor not liking the heat. take all the battery, starter and alt wires off, clean connections up till shiny then refit. may be worth upgrading the battery caBabling. mk3 golf +ve cables fit, chunky and better clamp design. -ve doesnt quite, need to add a short bit between earth stud and the ring on the mk3 cable. if you can find a b4 passat in scrappy some models o them have one which bolts right on.