Just picked up a Super Course for a good price (seems to be a rash of these lately).

I decided just for the heck of it to install a Shimano Light Action rear derailleur (the previous owner had 'updated' the simplex drivetrain to a suntour 'u' rd and a low-level shimano fd) and accompanying shifters.

My problem is that when in the big ring up front (on a double) and two largest cogs in the rear (on a 5 sp freewheel) I get some pretty serious cross chaining that throws the chain off the ring. I wouldn't really mind except that this eliminates 2 of my 10 speeds and seems fixable.

So here are my questions:
1) the Light Action rd is long cage where the Suntour 'U' was short--this couldn't be part of the problem, could it?

2) Does it have to do with chain length? The long cage effectively shortens the chain, but it was somewhat too long to begin with. Now, when in the big ring up front and smallest cog, the jockey wheels are perfectly aligned vertically.

3) Is this just a standard 10sp dilemma, something to put up with and forget about?