My trusty old 90 has just been serviced, and had now developed a fault...

When going uphill, drives fine as long as he revs are kept right up, as soon as I change gear and the revs drop, the engine dies.

Any thoughts?

It's still running the original 1988 engine and is due for a transplant, but didn't really want to do it just yet

Red one's don't go up hills. It's a known problem.